Safe room

Safe Rooms are the way most of the chapters of Left 4 Dead end: a barricaded, unbreachable safehouse for survivors to rest in between fending off hordes of infected. Safe rooms are easily identified by the big red door that they all have. Once all living survivors are inside the room and have closed the door and the next level will load. Most safe rooms are equipped with three things, First aid kits, ammo, and first tier weapons. The only exception to this are the safe rooms right before the finale of a level which are the same except they hold second tier weapons and, possibly, grenades.

Safe rooms are the only part of a map where survivors are totally safe. None of the infected can break down the doors, save for the Tank. But if the doors remain open, then any infected can enter the room until the door is closed. If an infected enters the safe room, it must be killed or shoved out of the room before the next level will load.