(covers information from several alternate timelines)
The shuttlebay (variously called the flight deck, hangar deck, or shuttlecraft bay) was a facility on a starship where shuttlecraft were launched, received, stored, and maintained.
The use of shuttlebays during the 22nd century (ENT: "Fight or Flight", "Cold Front", "Shuttlepod One", "Storm Front") and 23rd century (TOS: "The Galileo Seven", "The Doomsday Machine", "Journey to Babel", "The Immunity Syndrome", "Let That Be Your Last Battlefield") involved depressurization of these shipboard facilities, during which time the space doors would be opened to permit ingress and egress of spacecraft. By the 24th century, development of force-field technology allowed shuttlebay space doors to usually be kept closed, and when these were opened during shuttlebay operations, the physical doors were replaced with a force field where shuttlecraft or other space vehicles were about to board or were about to be launched. These force-fields could be penetrated by the shuttlecraft itself, while nonetheless preventing decompression through exposure to the vacuum of space of the shuttlebay. (TNG: "The Child", "Time Squared", "The Best of Both Worlds, Part II")
Most shuttlebays were equipped with a tractor beam emitter in order to facilitate the landing procedure. During the 22nd century, these facilities were referred to on Starfleet vessels as "launch bays" and used a grappler. (ENT: "Broken Bow", et al.)
On at least Constitution-class starships, flight decks were overlooked by an observation deck. The location of the hangar deck for this class was depicted on a schematic stored in the USS Enterprise-D library computer in 2364. (TOS: "The Conscience of the King"; TNG: "Datalore")
Constellation-class starships had seven shuttlebays. At the front of the ship, there was shuttlebay 1. Along the starboard, there were shuttlebays 2, 3, and 4. Along the port, there were shuttlebays 5, 6, and 7. (TNG: "The Battle")
Galaxy-class starships had several shuttlebays. A main shuttlebay was located in the primary hull while shuttlebays 2 and 3 were located in the secondary hull. These bays were supported by several hangars. (Star Trek: The Next Generation, TNG: "Evolution")
Deep Space 9's landing pads provided a similar storing location for small starships such as Danube-class runabouts, Ferengi shuttles, and Romulan shuttles. (DS9: "Emissary", "The Magnificent Ferengi", "In the Pale Moonlight")
Intrepid-class starships had a shuttlebay located at the rear of the secondary hull supported by at least one hangar, commonly referred to as shuttlebay 2. (Star Trek: Voyager, VOY: "In the Flesh", "Body and Soul")
Aircraft carriers of the 20th century were the first Earth ships equipped with hangar decks. These decks were used for the storage of aircraft and their accessories. (Star Trek IV: The Voyage Home)
