Also called the active roster, the 25-man is composed of 25 players who are playing for their Major League team. They are the starting eight position players, pitchers, and reserve players on the team. Players on the active roster are also on the 40-man roster. These players are generally the only ones who dress in uniform and are the only ones who may take the field in a game at any time. Typically, only players on the 25-man roster, and players who are on the disabled list but were on the 25-man roster, travel on road trips with the Major League team.
Typically, a 25-man roster will consist of five starting pitchers, seven relief pitchers, two catchers, six infielders, and five outfielders. In the American League a full-time designated hitter is usually classified as either an infielder or an outfielder, not a DH, because most DHs do play first base, left field, or right field from time to time (or occasionally a DH will be a catcher, like the Detroit Tigers DH Victor Martinez).
Beginning in the 2012 season, teams were allowed 26-man rosters for scheduled "day-night" doubleheaders—two games scheduled on the same day, but with the stadium cleared between games, and separate tickets sold for each game.
Read more about this topic: Major League Baseball Rosters