There are two Disabled Lists in major league baseball, 15-day and 60-day. To be placed on either DL, a player must be certified disabled by a doctor and may remain on either list for as long as required to recover.
Players may be disabled retroactively, up to a maximum of 10 days, beginning with the day after the last day they played. During this 10 day period, a player status is said to be day-to-day, indicating that the team is in the process of deciding if the player must be placed on the DL or if he is healthy enough to return to active service.
Injured players may not be traded without permission of the Commissioner nor may they be optioned to the minors, though they may be assigned to a minor league club for a "rehab assignment" for a limited amount of time (30 days for pitchers, 20 for non-pitchers).
15-day: There is no limit on the number of players per club.
Players on the 15-day disabled list are removed from the 25 man active roster, but are still a part of the 40-man roster and may not play for at least 15 consecutive days.
Players placed on the 15-day DL may be shifted to the 60-day list at any time, but not vice versa.
60-day: There is no limit on the number of players per club, but it may be used only when a club is at the maximum of 40 players.
If a player is transferred to this list, sometimes called the Emergency DL, after Aug. 1 he must remain there through the end of the season and postseason.
Players on the 60-day disabled list do not count against either the active 25-man or the 40-man roster and may not play for at least 60 consecutive days.