Baseball isn't just about hitting the ball and scoring runs. When the other team are batting, all of the hitters (except the Designated Hitter, if one is being used) have to field and support the pitcher. A player's skills in the field can be just as valuable as his hitting skills, though some positions are considered more important than others.
The better the fielders, the smaller the gaps available to the opposition hitters, and the greater margin of error for the pitcher.
Teams generally field with four infielders (in and around the diamond), plus three outfielders, along with the catcher and pitcher. These players are also known as "position players".
All fielders are allowed to wear a glove, which they use to catch the ball. They cannot catch a ball with their clothing, and the first baseman and catcher are allowed to wear bigger gloves than other players. The catcher will also wear various knee guards, chest pads and a face protector in case he's hit by a baseball.
The catcher stands just behind the home plate, about four feet behind a hitter swinging a bat, and is expected to catch the ball when t he hitter misses or doesn't swing at it (and even a few he does hit). The catcher normally calls the pitches the pitcher will throw, as well as having to quickly throw to second or third base to catch out runners trying to steal a base.
Catching is by far the most demanding fielding position physically, and most teams will carry two or even three catchers, to enable them to give the starter a day off regularly. Hitting ability is not considered a primary requirement for a catcher (it's a bonus) and usually a catcher will be found batting towards the bottom of the lineup.
The ability to "call a game" is also critical, reading the pitcher's rhythm, knowing which pitches to call and when. The catcher will make a signal to the pitcher for which pitch he wants (the pitcher can't signal, because the hitter is watching him) and if the pitcher doesn't like it you'll see him "shake him off", asking the catcher for a new signal. Sometimes you'll see a pitcher repeatedly shake off the catcher, in which case the catcher may well walk over and have a discussion with the pitcher about why he's calling what he's calling. It's not a good sign if the pitcher and catcher aren't in sync.
The first baseman protects the first base line, and has to normally cover first base (which is where most balls are thrown). It's rare for him to have to throw the ball (usually only to second base to execute a "double play") and he doesn't have to move much either. It's not uncommon for a team to "hide" an ageing, power-hitter at first base.
As the first baseman doesn't have to tag runners (he only needs to tag the bag) you'll often see him with one foot touching the bag, and stretching to reach an errant throw - if the throw is so far away he's "pulled off the bag" and won't make the out.
The shortstop is generally considered the key fielding position in the team, and until the 1990's it was rare for teams to worry about having a shortstop who could also hit. He normally fields in the gap between the second and third basemen and needs to be quick, agile and an expert at throwing to first or second base to catch out runners (and deciding which to throw to).
He also often serves as a relay man for throws coming in from the outfield - rather than outfielders throwing long arcing (and therefore slow) throws to one of the bases, they'll throw hard and flat to the "cutoff man" who then relays it at speed to a base. Throwing to the cut-off man also then gives the fielding side the option of making as late a decision as possible as to where the best chance of making an out may lie if there is more than one base runner, or preventing base runners from advancing on a fruitless long throw to home plate.
The second baseman is usually second in ability to the shortstop, and the two work closely together. More often than not, if the shortstop takes a day off, the starting second baseman will play short instead. Many of his skills are the same as the shortstop, but he also needs to be able to "turn the double play" at second, taking a quick throw from another infielder, tagging the bag and in the same motion "pivoting" and throwing to first base (often whilst a runner slides in to second to try and "break up" the double play). Often the shortstop and second basemen will swap roles on this (if the second baseman fields the ball, the shortstop will be the "pivot".
The third baseman probably sees the least of the ball of any of the infielders, but when he does see it, it's usually travelling quickly. He fields in the position where a right-handed hitter pulls a ground ball, and his position is often known as the "hot corner". He needs to have a powerful arm, as he is normally throwing "across the diamond" (from third to first) as well as good mobility, as most bunts are fielded by the third baseman.
Centre Field/Left Field/Right Field
The three outfielders are normally fairly interchangeable, and normally selected less for their fielding ability than for their hitting ability. The centre fielder is generally the best fielder, as he has the most ground to cover, coming in short to field any balls "blooped" over the short-stop and second baseman, or deep to field any balls driven to the wall (which is usually furthest away at centre field).
The left fielder tends to be busier than the right fielder, because he has to deal with deep balls pulled by right handed hitters, whilst the right fielder tends to need to have the strongest arm because of the long throw to third base.
Communication is key amongst the outfielders, as a collision between two outfielders racing for the same ball is one of the most dangerous aspects of all baseball. They also need to have strong arms, to get fly balls or ground balls thrown back into the infield as quickly as possible, as well as good range to track down a ball hit in the gaps which may go to the outfield wall on the ground. Leaping ability can also be a bonus - an outfielder is allowed to leap and reach over the wall to catch a ball which would otherwise be disappearing for a home run.
The centre fielder will sometimes creep in and field very shallow indeed, so that the defensive unit almost have five infielders and two outfielders.
Major League Baseball teams are normally allowed to have twenty-five players on their roster for each game, and will normally carry thirteen or fourteen "position players". In addition to the eight starters, a team will normally have a backup catcher, a backup outfielder (or two) and two backup infielders (or two). One of these may be the Designated Hitter if it's in use.
These players start the game as "the bench" and can be used as substitutes at any time, whether in case of injury, fatigue (rare for hitters), tactical reasons or because one of the starters has been thrown out of the game by the umpires.
Each position is assigned a number for scoring purposes:- Pitcher - 1, Catcher - 2, First Baseman - 3, Second Baseman - 4, Third Baseman - 5, Shortstop - 6, Left Fielder - 7, Centre Fielder - 8, Right Fielder - 9. For example, if the short stop fields the ball and throws to first base to tag out the runner it'll be referred to as a "6-3 out".
A double play occurs when the fielding team manage to get two outs in one play (at least one base runner must be involved). Most commonly it occurs when a runner on first base is forced out at second base, and the second baseman or shortstop then throws to first base to put out the hitter (the force at second is almost always made first, as that runner has a head start). A player may even be involved twice in the double play (first baseman fields the ball, throws to second, gets a throw back to tag the base) or even may complete the play unassisted.
If there is a runner on second and/or third base, but none on first, and less than two outs, fielding sides will quite often intentionally walk a hitter onto first base to "set up" (increase the chance of) the double play.
If the batting team has a runner on first base (especially if there is one out and a further runner in scoring position), the fielding team will often bring the infielders in to "double play depth" - hoping to field a ground ball early and get outs at both second and first base. The danger is that by fielding closer the hitter has more chance of hitting the ball through the infield - the fielding team offsets the increased risk of a hit to maximise the chance of making the double play.
A triple play is similar to a double play, but rather rarer, requiring no outs, and at least two base runners. Mostly commonly it involves a sharp line-drive to an infielder, with the ball then relayed to "double off" base runners who left early, and cannot get back to their bases, but may also involve a hit straight to a fielder on second or third base (who tags the bag on a force out), then throws to the other base and thence to first.
An unassisted triple play is also possible, but rare. It will usually involve a line drive being caught at second base (so the hitter's out), the baseman tagging second (where the runner may well have set off, not anticipating the catch, so he's not supposed to have left the base) and then tagging a runner who has set off from first to second (and has to turn, and go back to first).
The Infield Fly-Rule
If, with less than two outs, and runners on first and second, a fly ball is hit in the infield, the fielding side have an easy catch to make, and the runners don't have to advance. However, if the fielder were to "accidentally on purpose" drop the ball, all runners (who would be holding on base) would suddenly have to advance, and a force-out double play would become a likelihood. To prevent this unfair situation, the umpire will call "infield fly" when the ball pops up, and the hitter is automatically retired, even if the ball is dropped. The runners don't have to advance, unless these choose to do so, in which case they can still be tagged out as usual.
This does not apply to balls hit to the outfield, only to fly balls hit to an infielder where the fielding side could otherwise profit from an "accidental drop".