There is only one really meaningful statistic in Baseball. How many runs did you score? And was it more than the other team? However, Baseball is a game that lends itself to all sorts of hitting, pitching (and to a lesser extent) fielding statistics, and many of these are detailed here, along with the codes that often accompany them.
It should be noted that in Baseball many "percentages" are referred to, but the figures aren't expressed as percentages, but in fact as decimal fractions. That's tradition for you!
Games (G) - The number of games in which the hitter has actually appeared.
At Bats (AB) - The number of times in which the hitter appeared at the plate and made a base hit, reached base on an error, or was out. Plate appearances which led to a "walk" are not counted, at which a batter was hit by a pitch or awarded first base due to interference and nor are "sacrifices" (appearances where he intentionally sacrificed his chances of getting on base to allow base runners to advance).
Runs (R) - The number of runs scored by that player, by reaching home base in any manner.
Hits (H) - The number of base hits made by that player, not including reaching base on an error or on a "fielder's choice".
Total Bases (TB) - The total number of bases made by the player when hitting (singles count one, doubles count two, triples count three and home runs count four).
Doubles (2B) - The number of base hits made by the player that allowed him to reach second base.
Triples (3B) - The number of base hits made by the player that allowed him to reach third base.
Home Runs (HR) - The number of base hits made by the player that allowed him to reach home base (almost always a hit over the park's perimeter walls, though it includes "inside the park home runs").
Runs Batted In (RBI) - Runs scored due to a batter making a base hit, sacrifice, walk etc.
Note the difference between a Run and a Run Batted In. If the hitter makes a base hit, allowing a runner to score from third base, then the runner is credited with the Run, but the hitter is credited with the RBI.
If a run is scored because of a fielder's error, on a wild pitch, a passed ball, a balk or on a steal of home base, the hitter concerned is not credited with a RBI. This may mean that a run is scored but no RBI credited.
Base on Balls (BB) - A hitter is credited with a Base on Balls (commonly known as a "walk") when he advances to first base without a hit because the pitcher threw him four balls.
Hit By Pitch (HBP) - The number of times a hitter was awarded first base because of being hit by a pitch.
Sacrifices - A batter is credited with a sacrifice when he deliberately hits a ball that gets him out, but allows a base-runner to advance. A Sacrifice Bunt (SB) will normally advance a runner on first or second base, a "sacrifice fly" is credited when it allows a runner on third base to score.
A sacrifice does not count as an at bat, but the hitter may be "credited" with an RBI (so a hitter can be credited with a sacrifice and an RBI, but has no base hit and no at bat recorded).
Stolen Bases (SB) - If a runner manages to move from one base to another without the hitter hitting the ball (he runs while the pitcher is pitching the ball and beats the catcher's throw) then he is credited with a stolen base.
Caught Stealing (CS) - If a runner is caught out trying to steal a base he is charged with having been "caught stealing".
You will often see the two base stealing figures quoted together, usually as number of successful steals compared to the number of attempts. It's generally considered a successful base stealer needs to have around a 70% or better success rate.
Strike Outs (SO) - A strike out is recorded against a hitter when the pitcher retires him with three strikes. If a hitter "flies out" or "grounds out" this is not a strike out.
Fielding Errors (E) - If a fielder makes an error which prevents a hitter or runner being put out (drops a catch, fumbles a pickup, throws to the wrong base) or allows him to advance a base or bases then the fielder is "charged" with an error.
Batting Average (Avg) - Batting Average is calculated by taking the number of Base Hits and dividing by his number of At Bats. Batting appearances which end in a Base on Ball or a Sacrifice do not count.
.250 is par, .300 is very good and .350 is outstanding.
On Base Percentage (OBP) - On Base Percentage is similar to batting average but includes appearances which led to walks and times a hitter was hit by a pitch. It is calculated by adding Base Hits, Bases on Balls and Hit by Pitches, and dividing by the sum of At Bats, Bases on Balls, Hit by Pitches and Sacrifices.
Traditionally Batting Average has always been seen as the "measure of a batter", but more recently On Base Percentage is seen as a better guide. It doesn't matter how a runner gets on base, what matters is that he gets there!
.300 is par, .350 is very good and .400 is outstanding.
Slugging Percentage (SLG) - Slugging Percentage is calculated by dividing Total Bases by number of At Bats.
Anything over .500 is outstanding, a handful of players have career records over .600. Slugging percentage is the traditional measure of a hitter's "power".
On Base Plus Slugging Percentage (OPS) - is calculated by adding On Base Percentage and Slugging Percentage together.
OPS is the newest "statistic" and considered by many to be the best measure of a hitter's skill, combining both his ability to get on base and his "power statistics". Anything over .800 is very good.
Left On Base (LOB) - Usually shown as a team statistic, this shows the number of base runners "stranded" when an innings is completed.
Putouts (PO) - Whenever a fielder retires a batter (catching a fly ball or tagging out a runner) he is credited with a putout.
Assists (A) - Whenever a fielder assists in retiring a batter he is credited with an assist (e.g. if the shortstop fields the ball and throws to first base where a runner is tagged out then the shortstop is credited with an assist and the first baseman a putout).
Fielding Percentage (FPCT) - Fielding percentage is calculated by adding up putouts and assists, and dividing by the sum of putouts, assists and errors. Fielding percentage should normally be expected to be above .950 (95%).
Double Plays (DP) - The number of Double Plays in which the fielder participated. One (fielder makes a catch, and puts out a runner himself), two (fielder makes a catch, throws to a team-mate who forces or puts out a runner, or fielder throws to a team-mate who puts out a runner, and then throws back to the original fielder who puts out a second runner), three (fielder throws to a team-mate who puts a runner, and throws to a third team-mate who puts a second runner) or even more (if a deflection or rundown is involved) fielders can be credited in any given double play.
Note that only one double play is added to the team total, so the team total for double plays will usually be about a third of the sum of the individual players' totals. Also note that a double play cannot occur if there are already two outs in the inning, as the first out of the play ends the inning.
Triple Plays (TP) - Very rarely a team can turn a triple play, getting out three runners in one play. Any number (usually no more than four) of fielders may be involved (but a solo triple play is possible, albeit very very rare).
Range Factor (RF) - Range Factor is another "modern statistic", and is calculated by the number of Putouts and Assists combined, divided by the number of innings played by the player. It is therefore claimed to be a measure of how frequently the player fields the ball (or perhaps how frequently it's hit in his direction!).
Passed Balls (PB) - The number of "passed balls" allowed by a catcher (a pitch he should have caught, but fumbled and allowed base runners to advance).
Games (G) - The number of games in which the pitcher has appeared.
Games Started (GS) - The number of games the pitcher started.
Complete Games (CG) - The number of complete games a starter has pitched (a game he started and finished, and which went for at least nine innings).
Games Finished (GF) - The number of games in which the pitcher finished the game (i.e. the last pitcher in the game).
Innings Pitched (IP) - The number of innings pitched. A pitcher is credited with a third of an inning when he gets one hitter out. It is possible for a pitcher to play for "zero" innings, e.g. if he comes in, gives up three hits and is promptly benched.
Hits (H) - The number of base hits given up by the pitcher.
Runs (R) - The number of runs scored off the pitcher, including those due to errors by the fielding team.
Note that when a pitcher is lifted from the game he retains responsibility for any runs that may be scored by base-runners left behind by him when he was replaced (or indeed any runners who get on base as a consequence of "fielder's choices" erasing runners left by him but allowing a new hitter to reach base), so if he leaves the game with a runner on second base, and that runner subsequently scores, the run is "charged" to the original pitcher (because he put him there).
Earned Runs (ER) - The number of "earned" runs scored off the pitcher, which do not include runs which are scored as a result of a fielding error by the pitcher's team.
Whether a run is earned or not is quite complicated. If a runner got on base because of an error, or if the inning is only still in progress because of an error (i.e. there should have already been three outs and the inning over, but an error meant one of the "outs" wasn't made) or if the runner advances as a consequence of an error, then the run is "unearned" (i.e. not considered to be the fault of the pitcher).
As a general rule of thumb you'll find that about 90% of runs are earned runs. Note also that a pitcher can be responsible for the fielding error himself, and the run still remains "unearned".
Home Runs (HR) - The number of home runs scored off the pitcher.
Bases on Balls (BB) - The number of "bases on balls" (walks) issued by the pitcher. This does not include Intentional Bases on Balls (where the manager effectively instructs the pitcher to walk a batter for tactical reasons).
Intentional Bases on Balls (IBB) - The number of "intentional bases on balls" (walks) issued by the pitcher.
Strikeouts (SO) - The number of hitters "struck out" by the pitcher.
Wild Pitches (WP) - The number of "wild pitches" thrown by the pitcher (which got by the catcher, allowing base runners to advance, and determined by the Official Scorer to be the pitcher's fault, not the catcher's).
Balks (Bk) - The number of "balks" called against the pitcher, whereby he illegally interrupts his throwing motion and the runners are awarded an extra base.
Won-Lost Decisions (W-L) - In Baseball a "win" and a "loss" is always credited/charged to one pitcher on each team. A win is credited to a starting pitcher if he held the lead when he left the game and his team never subsequently lost the lead, or to the pitcher who was the "pitcher of record" when the winning run was scored (the last pitcher who got the last out when his team scored the winning run).
Example 1: a starter pitches six innings and leaves with a 3-1 lead. His team extend the lead to 5-1 in the eighth but the opposition score two runs in the ninth so the game finishes 5-3. The starter is awarded the win because his team always led once he left the game.
Example 2: a starter pitches six innings and leaves with a 3-1 lead. The opposition tie the game at 3-3 in the seventh, and his team extend the lead to 5-3 in the eighth and the game finishes 5-3. The win is awarded to whichever pitcher got the final out of the seventh inning (because he was the "pitcher of record" at the time the winning run, the fourth, was scored).
A starter has to pitch at least five innings to be credited with a win. If the starter would normally have been credited with a win, but has not pitched five complete innings then the win is awarded to the "most effective" relief pitcher in the opinion of the Official Scorer.
In the same way, if a starting pitcher leaves the game with his team trailing, and his team never manages to tie or lead the game, he is charged with a loss (he doesn't need to have pitched a minimum five innings!). Otherwise a loss is credited to whichever relief pitcher on the losing team was the "pitcher of record" when the winning run was scored (so if a team loses 6-3 and the starter isn't charged with the loss, it's whichever relief pitcher gave up the fourth run).
If a pitcher completes an inning then he remains the "pitcher of record" until he is actually replaced at the start of the next inning, so any runs in his team's next half inning (i.e. before he is replaced) are charged to him.
Notes: Wins and Losses are fairly arbitrary. It's quite possible for a pitcher to only allow one run in nine innings, yet lose a game 1-0. Equally, he can allow nine runs in five innings, leave with a 14-9 lead and get the win. Or a relief pitcher can come in with a 5-0 lead, allow five runs and then see his team-mates score one run to win it 6-5, and he gets the win. Over a season these things usually even out, but Earned Run Average (see below) is often considered the better measure of a pitcher's value.
Run Support (RS) - The number of runs scored per nine innings by the pitcher's team-mates.
Saves (Sv) - A premium is placed in baseball on the pitcher who comes in at the death, and manages to protect a narrow lead to preserve a win. Any time a closing pitcher (the last pitcher to play in the game for his team) enters a game in which his team leads by three or fewer runs, or with the tying run on base, at the plate, on in the "on deck circle" (i.e. the next hitter after the one at the plate) and finishes the game without giving up the lead he is credited with a "save". A save may also be credited to a closing pitcher who "pitches effectively" for at least three innings (in the opinion of the official scorer).
Note: a save is not credited in every game, if a game is a blowout then there won't be a save, it's only awarded when a pitcher inherits a tight lead, and keeps the lead.
Save Opportunities (SvO) - The number of opportunities a pitcher had to make a save. "Blown saves" are rare, and a good closer is expected to "convert" at least 90% of his Save Opportunities.
Note: Saves are of varying difficulty. It's much easier to come in with a three run lead, no runners on base and one out required than to come in with a one run lead, bases loaded and no outs. Nonetheless, both are "save opportunities".
Hold (Hld) - A hold is credited to a relief pitcher who does not finish the game, but enters the game with a lead, and leaves a game with the lead intact. Effectively it's a statistic for "middle relievers".
Earned Run Average (ERA) - Earned Run Average is expressed as the number of Earned Runs allowed by a pitcher per nine innings, the standard length of a game (so ERA is calculated by dividing the number of Earned Runs given up by the pitcher by the total number of Innings pitched and then multiplying by nine).
Walks and Hits Per Innings Pitched (WHIP) - Is calculated by adding the number of "bases on balls" issued and hits allowed, divided by the number of innings pitched. It's a good measure of how effective a pitcher is at keeping runners off the bases.
Note: WHIP is a relatively "new statistic", becoming increasingly popular. Anything below 1 is consider to be good.