A hangar is an enclosed structure for housing aircraft. There is a disagreement as to where the term originally evolved, but most information points to the original German Zeppelin airships that were "hung" inside large barn-like buildings, which were nicknamed "hangars."
Hangars are like big garages, only instead of cars, they store aircraft. A hangar protects the aircraft from the elements, allows mechanics a more suitable environment to work on the aircraft, and keeps the aircraft safe from theft. Military hangars also hide the aircraft from enemy reconnaissance aircraft or spy satellites, so that the aircraft design or location can be kept secret.
Most hangars are constructed like a barn, with a high ceiling to fit an airplane’s tail and wide doors to fit the wings or rotors. A large door can usually be rolled out of the way or lifted above the airplane to allow storage or to move the aircraft from the hangar. A light hangar is often constructed of wood or metal with a concrete or gravel floor, while larger aircraft hangars are assembled from reinforced concrete to bear the load of such a large building. Military hangars are typically reinforced with concrete to prevent damage to the aircraft in case of an explosion or attack.
Cutaway diagram of an Aircraft Hangar: