NFL rules
A team may place a player on injured reserve (reserve/injured list) who is "not immediately available for participation with a club". Generally, these players may not practice or return to the Active List for the rest of the season (including postseason games or the Pro Bowl) in which they are placed on injured reserve, but are allowed to be with the team.
 
During the preseason, the league also allows players with long-term, but not season-ending, injuries to be placed into one of two designations: physically unable to perform (PUP), for injuries sustained during the previous season or during training camp, or reserve/non-football injury, for injuries sustained outside of team or league activities (despite the name, this includes lingering injuries from college football play, should an injured player be drafted and join the team). Both designations are functionally identical in that the player can be promoted to the active roster by week 6 of the regular season or placed on injured reserve.
 
Starting in 2012, the NFL and the NFLPA reached an agreement allowing one player placed on injured reserve to be brought back to the active roster.Provided that the player was on the final 53-man preseason roster (A rule exempted for the 2012 season), and that the injury is deemed to keep this player unable to practice or play football for an estimated six weeks, the player may be allowed to practice after Week 6, and be activated to play after Week 8.

 
Teams may also place a player on injured reserve with a minor injury designation, but the team must release the player once he is healthy

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