Each Major League Baseball team maintains both a 25-man roster and a 40-man roster of players.
Also known as the active roster, it 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. 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 and players who are on the disabled list but were on the 25-man travel on road trips with the Major League team.
The 40-man roster includes the players on the 25-man roster plus as many as 15 players who are either on the team's 15-day disabled list or who are in the team's minor league system. From September 1 through the end of the regular season, any player on the 40-man roster (also referred to as the "expanded roster") is eligible to play in an official regular season game. Many young players make their Major League debuts in this way, as "September call-ups." For non-playoff bound teams, these are players from the minors who are called up to get some Major League experience.
Players must be on a team's 25-man roster as of August 31 to be eligible for post-season play, with a couple of exceptions. Basically, to be eligible for a team’s playoff roster, a player must be on either a) the 25 man active roster, b) the 60-day disabled list, c) the bereavement list, or d) the suspended list as of August 31st at midnight.
On the day before deadline, here is the Rays' current 25-man roster.
Tampa Bay Rays Roster