An emergency exit in a structure is a special exit for emergencies such as a fire: the combined use of regular and special exits allows for faster evacuation, while it also provides an alternative if the route to the regular exit is blocked by fire, etc.
It is usually a strategically located (e.g. in a stairwell, hallway, or other likely place) outward opening door with a crash bar on it and with exit signs leading to it. The name is a reference to when they are frequently used, however a fire exit can also be a main doorway in or out. A fire escape is a special kind of emergency exit, mounted to the outside of a building.