From BR Bullpen

The infield is the portion of the playing field delimited by the three bases and home plate. It forms a perfect square with 90-foot sides, with the bases and home plate located at the four corners. Because the square appears to be standing on one of its points, it is often refered to as a diamond, hence the term baseball diamond used to refer to a playing field. The infield is entirely in fair territory. The area beyond the infield is called the outfield.

There are six players who have fielding responsibilities in the infield; however the pitcher and the catcher are not usually refered to as infielders because their duties are so specialized. The proper infielders are the three base players - the first baseman, the second baseman and the third baseman - and the shortstop, who covers the area between the second baseman and the third baseman.

While the infield is properly only the area within the diamond, in many cases it also includes an area delimited by a semi-circular line situated some 100 to 120 feet from home plate. This line may or may not be demarcated on the field. This reflects the fact that the infielders normally play some distance behind the bases - except in situations when they want to prevent a baserunner from scoring from third base with less than two outs, in which case the infielders will play in. In the case of the infield fly rule, this larger definition of the infield is used by the umpires to determine whether a pop-up hit in an infield fly situation is an automatic out or not.