Scoring and Base-Running
The easiest way to score a run is to hit a home run! If the batter hits the ball out of the park (within the fair territory lines) then he gets to trot around all of the bases and scores a run, and any other base runners also score. However, only 10-15% of hits are home runs, so teams shouldn't rely on the "long ball" to score runs (though some do)!
A home run has to be hit "on the fly" - if the ball bounces out of the park then it's known as a "ground rule double" and the runner is automatically awarded second base (a similar applies if a fan reaches over the fence and grabs a ball, preventing a fielder from playing it - the umpires will award a number of bases according to what they think would have happened without the "fan interference").
One tradition in baseball is that on a home run, the hitter doesn't "show up" the pitcher by celebrating - he simply trots around the bases with no exuberance. If he overdoes any celebration, he (or a team mate!) can expect to be hit by a pitch next time he comes up to bat.
Singles, Doubles, Triples
There are four sorts of hits:- singles (where the hitter reaches first base), doubles (second base), triples (third base) and home runs. Triples are rare (it's difficult to keep the ball in the park and have enough time to run 270 feet, though possible, and even "inside-the-park home runs" are possible), doubles are reasonably common (around 20% of hits, usually into the outfield) and singles are the most common of all.
Walks (Base on Balls)
If a hitter is "walked" (or awarded first base if he's Hit By a Pitch) and there is already a runner on first base, then that runner moves from first to second base. If second base is also occupied, then that runner moves from second to third. If third base is also occupied (i.e. the "bases were loaded") then the runner on third base walks to home and scores a run. It's never good for a pitcher to "walk in a run".
A team generally gets less than half as many walks as hits during a game, although the modern game places more of a premium on getting on base, and walks are more common.
Advancing the Runners
When a hitter hits the ball, base runners will usually try and advance, but don't have to. If a runner is on first base then he must advance, as the hitter is running towards first - the runner on first is said to be "forced" to advance. Similarly, if there are runners on first and second then both are "forced" to run, but if there's only a runner on second and none on first, the runner on second cannot be "forced".
Base runners have an advantage over the hitter - they aren't standing in a batter's box swinging a bat, and can take off as soon as the ball is hit (or more commonly, beforehand). A base-runner is allowed to get a head-start before the pitch is thrown but the pitching team can try and "pick him off" instead of throwing to the hitter. It's common to see a base-runner "taking a lead" (inching away from the base) and the pitcher throwing over to first base making him dive back. Threatening to steal a base not only makes it easier for base-runners to advance, but also distracts the pitcher from his duel with the batter. Furthermore, it makes the first baseman stand closer to the bag ("holding the runner") rather than standing where he'd ideally like to be fielding.
Scoring a Run
To score a run, a runner must not only touch home base, but he must do so before the third out of an inning is made. If the hitter or any forced runner is put out at the base he is running to, and this is the third out, then the run does not count, even if the "scoring" runner touches home base before the subsequent out is made. However, if a runner attempts to make an extra base, or isn't forced to run, then if he is tagged out after the scoring run has been made, the run still counts.
A base runner on second (or better, third) base is in what is referred to as "scoring position". Namely that any sort of base hit (even a single) is likely to be enough to get him safely to home (remember he has a running start, and home plate is furthest away from the outfield). A runner from first base will often be able to score on a double, but it may be tight if he doesn't have great speed.
Stealing a Base
A base runner doesn't have to wait for a hit to try and advance a base. He can attempt to steal a base at any time, by "taking a lead" before the pitch, running as soon as the pitch is made, and getting to the next base before any throw tags him out.
A steal is not permitted if the hitter hits the ball foul (it has to remain in play for a steal to be legal) but a hitter will often deliberately swing and miss at a pitch (giving up a strike) to try and put the catcher off as he tries to throw out the attempted steal.
Steals are most commonly made from first to second base, partly because it's easier to steal (the catcher has further to throw from home plate to second base, than from home plate to third base) and partly because the priority is to get the runner into scoring position. A further advantage of stealing from first to second is that it reduces the chance of a double play. If a runner is on first base, then any ground ball gives a good chance of "hitting into a double play" where the fielding side get out men at both second and first base. If the runner is on second base (so he isn't forced to run when the ball is hit) the chances of a double play are much smaller.
Sometimes a runner will steal third base, and very rarely can even steal home.
If the fielding side manage to get the runner "caught stealing" (by tagging him before he gets to base) then he's one of the three outs required to end the inning. Teams generally want at least a 70% success rate to make attempting a steal worthwhile.
If runners are on first and second, the fielding side may even attempt a "double steal", sending both runners to try and confuse the fielding team, and guarantee at least one successful steal.
A runner isn't only restricted to running on fair hits. If a ball is hit and caught (usually referred to as a "fly out" if it's caught in the outfield, or a "pop out" if it's caught in the infield) then he is still allowed to run, but isn't allowed to leave his base until the ball is caught.
On outfield fly-balls it's common to see a runner "tagging up" and advancing a base (usually third to home or second to third) by beating a throw in from the outfield (if he doesn't beat the throw, he'll be tagged out) and occasionally a runner will do the same on an infield pop-out. If a runner is on third base and the hitting team has less than two outs they may even deliberately hit a "sacrifice fly" to score the runner.
If a runner does leave his base before the ball is caught, he has to get back to base and touch it or he can be tagged out - sometimes you'll see a player "doubled up" in this manner when he gambles on a ball not being caught, and can't get back to his base, or simply miscalculates and leaves his base too early.
Another form of sacrifice is the sacrifice bunt. Here the hitter deliberately "lays down a bunt" (blocks it - by holding the bat on the barrel close to the point of contact) the ball just feet from where he stands. The objective is to ensure that base runners can advance but the hitter has no chance of making first base himself. It's most commonly performed by hitters who aren't very good anyway and to advance a runner from first to second (though sometimes from second to third). If the fielding side mess it up, the hitter may still "reach safely", but usually he doesn't.
The most exciting sacrifice bunt is the play to get a runner to home base (to score), known as a "squeeze play", and the most extreme version of that is the "suicide squeeze" where the runner from third sets off at full speed as soon as the pitch is made, not even waiting to see if the hitter manages to bunt the ball - if he doesn't and it's a strike caught by the catcher, it's "suicide". If he waits until he's seen that the bunt has been laid, it's known as a "safety squeeze".
Note that, unlike a normal swing, a hitter cannot "foul off" a third strike with a bunt. If he tries to bunt the ball on two strikes and hits the ball foul, he's out anyway.
Often a fielder will have a choice to make about who to get out. For example a fielder may allow the hitter to reach first base in order to throw out a runner going to second, or third, or home (better to erase the "lead runner"). This is known as a "fielder's choice", and the hitter concerned is not credited with a base hit (because he would have been thrown out at first if the fielder didn't elect to throw out someone else instead).
The fielding side will often try and turn a fielder's choice into a double play.
Passed Balls, Wild Pitches, Throwing Errors
There are yet more ways for runners to advance around the bases. If the catcher lets a pitch get away from him (usually it runs to the wall behind him, the "backstop") then a runner can advance. A "passed ball" is when the scorer considers it the catcher's fault the ball got past (i.e. he should have caught it), whereas a "wild pitch" is when the scorer considers it the pitcher's fault (it was so inaccurate the poor catcher didn't have a chance of reaching it).
Runners can also advance on throwing errors, if a fielder "throws a ball away" and it doesn't go to the team-mate he intends, any runners are allowed to advance as far as they wish whilst the fielding side retrieves it.
To get a base runner out the fielding side normally have to "tag him", touching him with the ball (or with the glove holding the ball). However, if a base-runner is forced (i.e. any runner heading for first, a runner heading for second because the hitter is heading for first, etc) then they simply need to "tag the bag" by standing on the base with the ball in hand. Often when a runner is forced from first the fielding team will try and make a "double play" - putting out the runner at second by stepping on the base and then throwing to first to get the hitter out as well. Usually, the runner heading for second will slide in as wildly (but legally) as possible trying to "distract" the player trying to "turn the double play" by making him run the risk of being flattened by the sliding runner!
Except when running to first base (when he's allowed to overrun the base, so long as returns to it immediately and doesn't take off for second), a base runner is only safe when he is actually touching the base, which is why runners will often slide into second or third base - first it's more difficult for the fielder to tag them (because they're at ankle height) and second they can touch the base quickly and then keep contact with it as they slide to a stop. Only if the base is going to be comfortably reached will a runner "stand up" as he reaches the base (often of course, he'll try and run to the next base instead).
Remember that a runner is allowed to run back to a base, often you'll see a runner take a few steps past a base before realising he won't make it to the next base and return.
Base running is a skill. Not only does it require speed from the runner, but an instinct on how much of a lead he can take before taking off, when to take off, and judgment of when he can take an extra base. Teams normally have coaches at both first and third base to tell runners whether to try for an extra base or not (and woe-betide the runner who ignores a "stop" instruction and gets tagged out going for the extra base).
Base runners are only allowed to run along the "base paths", dirt areas marked out between the bases. If they run outside the base-paths (to avoid a tag) then they can be "called out". Nor are they allowed to deliberately hinder the fielding side (for example, knocking the ball out of a fielder's hand).
Sometimes you will see a runner get caught in a "run down" - for example he sets off to steal second base, realises he won't get there and tries to get back to first, and you'll see the first and second basemen throwing the ball to each other to try and make a tag without the runner managing to break past one of them to make it safe to a base. Occasionally the fielding side will make a mess of this and a runner escapes to reach base safely.
Awarding Extra Bases
If the thrown ball misses its target and goes out of play ("thrown away") then all runners are normally awarded two extra bases (and may score, if home base is awarded). This most commonly happens on an errant throw to first or third base which rolls into the dugout or goes into the stands. If a pitcher throws the ball away trying to throw out a base stealer then only one base is awarded.
Blocking Home Plate
Whilst a catcher used to be able to block Home Plate by standing (or crouching) over it whilst waiting for a throw, this is no longer allowed due to the number of serious injuries. Instead the catcher will try and tag the runner before he reaches the plate, though he may try to get away with obstructing the plate. Remember that the runner still has to touch home base, so at the same time as he's sliding in and wiping out the catcher he still has to make contact with the plate! And if the catcher is to make the tag, he has to hold onto the ball while he does so.
The "Grand Slam" is the most exciting play in baseball. "Bases loaded" (runners on first, second and third) and the hitter blasts a home run, scoring four runs in one swing.
Hit and Run/Pitchouts
On the "hit and run" play a base runner sets off as soon as the pitch is released, hopefully causing a fielder to move towards a base to cover the throw from the catcher. The hitter then tries to hit the ball through the gap just vacated by the fielder. If the runners set off too early, or if the hitter fails to make contact with the ball, the runner may well be thrown out easily. If the pitching team read the hit and run, the pitcher may even "pitch out", throwing the ball wide of the hitter to allow the catcher an easy chance to throw out the base runner (the catcher will usually stand up ready to start the throwing motion even as the pitchout is made).
There are many different philosophies on how to optimise scoring in Baseball. In the National League, where teams have to cope with a lineup including the pitcher, it's quite common to see what is referred to as "smallball". If the lead runner gets on base, he tries to steal second, the next hitter may (sacrifice) bunt him over to third, and then the next hitter may (sacrifice) fly him to score the run. It provides a high percentage chance of scoring a single run, but by sacrificing a couple of outs not much chance of many more. A run scored by "smallball" is often referred to as having been "manufactured".
In the American League, there's often more of a premium on the "big inning". Hitters and runners aren't sacrificed by bunting, risking steals and so forth, and whilst teams are less likely to score a run in an inning, if they do score they're more likely to score a bundle.