In Major League Baseball, the disabled list (DL) is a means for teams to remove their injured players from the roster in order to summon healthy players to temporarily replace them. There are two types of DL: the 15 day and the 60 day (previously there was also a 21 day DL). Players may be put on either list and may not rejoin the team until the associated number of days has elapsed. However, a player's time on the DL may exceed the specified number of days.
Sometimes a team will keep an injured player on the roster but listed as day to day meaning the medical staff is unable to determine when the player will be ready for play time and does not want to risk losing the player for two weeks with what might turn out to be a minor injury.
Players may be put on the DL at a date retroactive to the last day he played. This allows a team not to be penalized, beyond the possible disadvantage of playing with a reduced roster, for listing the player as day to day until the gravity of the injury is known.
The advantage to placing a player on the DL versus keeping him on the bench, is that it opens up a spot on the active roster, in which another player, usually either another player who has recovered from an injury or a minor league player, can be placed. The advantage inherent to the 60-day DL over the 15-day is that a player on the 60-day list does not count on the team's 40-man roster.