Hitting and Getting on Base
To score runs a team has to hit the ball (usually). A hitter will not swing at every pitch, usually he's waiting for a good pitch to hit, and this isn't as easy as it might sound. On average a hitter will only hit safely 25% of the time, and a hitter who can hit over 30% is likely to be a superstar. Bearing in mind that only 25% of swings will result in a safe hit, a walk to first base (for receiving four "balls") becomes quite an attractive proposition.
Why is Hitting so Hard?
To get a hit "all" a hitter has to do is hit the ball on the ground and far enough away from a fielder that he has time to run safely from the plate to first base. When he hits the ball, he isn't in a starter's blocks ready to sprint to first base - he's still completing his swing from having hit the ball.
Any runner who can cover the 90 feet from plate to first base in around 3.5 seconds is considered to be very quick, 4-5 seconds is far more typical.
Not only does a hitter have all of this to cope with, but if he hits the ball fair he has to run, and he cannot wait forever for an easy pitch to hit. Three strikes and he's out!
Hitting for Average/Hitting for Power
Most hitters tend to fall into one of two groups. Batters who "hit for average" concentrate on making base hits, usually singles (a hit which gets him to first base only) to maximise their chances of getting on base, whilst batters who "hit for power" try to make extra base hits (a double, which gets him to second base, or a triple, which gets him to third base) and home runs, usually at the expense of average.
A successful team generally needs a mixture of these hitters.
Left Handed, Right Handed, Switch Hitting
Most hitters are either right handed, or left handed. Generally right handed hitters are much more effective against left handed pitchers, and vice versa. However, some hitters can hit left or right handed, depending upon what the pitcher does - these are known as "switch hitters".
Occasionally you'll see a hit which ordinarily would not allow the hitter to reach first base, but because the fielding side elect to throw to a different base to get out another runner, they do not have time to make the out at first base. This is referred to as a "fielder's choice" and the hitter is not credited with a base hit (it's scored as if he hit an out, it's just that it was a team-mate who got out).
It is not a fielder's choice if the hitter hits safely, but a team-mate gets himself out on a running error.
Most good hitters are patient. They wait for the good pitch to hit rather than just swinging away at the first one thrown at them. Often they'll "take a pitch" they think will be just outside the strike zone to get a "ball". Ideally they want a pitch "in the middle of the strike zone", though the pitcher will usually be trying to aim the ball at the edges of the strike zone ("painting the corners"). If the hitter swings at everything, then the pitcher will probably throw the ball outside the strike zone anyway, and rely on the hitter "swinging and missing" for the strike.
What sort of pitch the pitcher will throw will normally be determined by "the count" (the number of balls and strikes).
e.g. a hitter has three balls and no strikes (a 3-0 count) - the pitcher knows one more ball and the hitter gets a "walk" and is likely to try and throw a strike (this is known as a "hitting count", when the percentages in favour of the hitter are much higher. Indeed, it's even possible that rather than risking a big hit, on a hitting count the pitcher will "intentionally walk" the hitter by throwing another ball).
At the other extreme, if the hitter has no balls and two strikes (a 0-2 count) then the odds are in favour of the pitcher. The hitter cannot "take any pitches" close to the strike zone, and has to try and hit anything close, though he may try and "foul it off" - if he makes contact but the ball doesn't go fair, it can't be strike three. This is commonly known as "fighting the ball off".
Each "at bat" is therefore a mini-contest in itself, with the odds constantly shifting. If the hitter can get ahead in the count, the pitcher's "strike zone" effectively gets smaller (because he has to throw a strike), if the pitcher gets ahead then the strike zone effectively "expands" (because the hitter has to chase anything close to a strike).
If a hitter reaches three balls and two strikes, this is known as a "full count". On a full count you'll frequently see the hitter fouling off the ball ("fighting it off") waiting for a good pitch to hit and keeping his "at-bat" alive.
If a force play at first base is not in operation, and a third strike is not caught by the catcher, then the strikeout is not called, and the hitter can attempt to run to first base (though usually he'll be tagged or thrown out). Note that the pitcher still gets a strikeout credited to his personal statistics, even if the hitter manages to reach first base without being thrown out.
Other Ways of Reaching First
Aside from a successful hit (known as a "base hit") or a walk ("base on balls") a hitter can also reach first base on a fielder's error, if he's hit by a pitch (usually he'll try and get out of the way, but sometimes he can't avoid a pitch, or doesn't even try - "taking one for the team" - though the umpire is required to rule that he didn't deliberately allowed himself to be hit) or if a fielder is called for "obstruction" (stopping the hitter running to first base).