From BR Bullpen
The disabled list or DL is the place where teams inscribe a player's name when he is unavailable for a lengthy period of the season due to an injury. This allows the team to replace the player on its roster with another player. When a player is on the disabled list, he cannot be used in a game, although he can stay with his team. He can even sit in uniform in the team's dugout with permission from the League. If the injured player is ready to return to action before the season ends while the team's roster is full, another player's name must be removed from the active roster to make room or the player coming off the DL must be waived (see waivers). Major League players are paid a Major League salary while they are on the DL.
Disabled lists are usually referred to by the minimum number of days players must be inactive. There are currently three types of disabled lists in Major League Baseball. The more commonly used disabled list is the 15-day disabled list. A player on the 15-day disabled list continues to occupy a spot on the Major League roster (i.e. the 40-man roster) but not on the active roster (i.e. the 25-man roster that limits teams till September 1st). Players must remain away from action for at least that number of days while they recover from whatever injury is preventing them from playing; they can however stay on the list indefinitely until they heal or the season ends. An injured player cannot be removed from the 25-man roster by being sent to the minors - his team must place him on the DL until he is healthy or the season ends. While they are on the disabled list, players may be sent, with their permission, on a rehabilitation assignment in the minor leagues. The length of these assignments is limited to a week or so, and players continue to be paid a Major League salary while they are on a rehabilitation assignment. This list was created in 1984 to replace the 10-day disabled list.
The second type of disabled list is the 60-day disabled list. A player on this list can be replaced both on the active roster and the Major League roster during the regular season and post-season. However, he must return to the Major League roster during the off-season if his team wants to retain his services for the future. This is used for more serious injuries that require many weeks of healing. Some players who are in fact retired are put on the 60-day disabled list for insurance purposes; this was the case of Albert Belle, who spent two full seasons on the Baltimore Orioles disabled list after announcing his retirement in spring training 2001 because of a degenerative hip injury. This move allowed the Orioles to claim reimbursement for a portion of Belle's guaranteed salary from an insurance company, but it also meant that they had to put Belle's name back on the Major League roster after the 2001 and 2002 seasons, thus losing a spot for an actual player.
The third and newest type of diabled list is the 7-day disabled list. This disabled list is specially made for players who have suffered concussions. Each team must designate a specialist in mild brain trauma to evaluate players and then send reports to MLB's medical director for approval before placing a player on this DL.
A player's name can be put on the disabled list retroactively, dating back to the last day in which he took part in a game, but limited to five days. This is often used in the case of pulled muscles, when the healing period, which is normally only a few days, sometimes stretches longer than anticipated. It is also used when a starting pitcher must miss a start with a minor injury; this allows his team to call-up a replacement starter, and brings forward the pitcher's return date by a few days. A player already on the 15-day DL may be moved by his team to the 60-day DL to free up a space on the 40-man roster.
The Disabled List was first instituted during the 1915 National League season, for a ten-day period at the time. Prior to 1990, there was also a 21-day disabled list as an intermediate range between ten and sixty days. Before the elimination of the 21-day list, the number of players who could be placed on the disabled list was limited, and there was much less flexibility about when they could return to action. For example, players with Major League contracts were not allowed to go to the Minor Leagues for rehabilitation purposes in those days.