A door was a moving structure that was used to allow or block off access between spaces. They were also critical components of airlocks. They were in use by numerous civilizations and there was tremendous variety. In size they varied from designed to accommodate humanoid movement to doors blocking starships from exiting spacedocks.
The doors at the Vulcan monastery at P'Jem, which was built around 3,000 years ago, were made of wood. The entrance door was a double-sided large door. Upon the arrival of the landing party from Enterprise NX-01 in June of 2151, the door featured several scratches and damage. The other doors inside the monastery were also made out of wood except one hidden single circular door found in the reliquary which was made of metal and defined the entrance to the secret listening post. (ENT: "The Andorian Incident")
Earth research modules were equipped with large circular doors which reminded the scientists to follow the contamination protocol. (ENT: "Regeneration") Similar, emergency instructions were also placed on the doors aboard the Earth Cargo Service freighter ECS Fortunate. (ENT: "Fortunate Son")
Starfleet doors normally had labels indicating the location of the room, the deck and the room number. (TNG: "Encounter at Farpoint", "The Outrageous Okona", "Chain of Command, Part II", "Genesis" et al.)
Groppler Zorn of the Bandi had the entrance of his office equipped with a door made out of wood. The door was destroyed by Commander William T. Riker and Data when they forced the entry. (TNG: "Encounter at Farpoint")
When Worf and Data beamed aboard the space module and Worf tried to pass a door but was unable because it didn't open for him. Data gave Worf a hint and pushed the door latch. (TNG: "The Neutral Zone")
Doors aboard Starfleet vessels were typically designed to open upon sensing the presence of an individual or individuals approaching them and then close again once they were out of range. In 2367, Captain Picard attempted this with the doors of his quarters upon the unexpected appearance of Ardra, who was attempting to seduce him. The doors, however, failed to open and he pressed a button on a control interface next to them, also to no effect. Ardra then stated that it was just the two of them. (TNG: "Devil's Due") Ensign Bradward Boimler, having returned to the USS Cerritos in 2381, found that the computer refused to recognize him and stubbornly went into a Jefferies tube to reach the bridge when doors wouldn't open for him. (LD: "We'll Always Have Tom Paris")
In 2369, Commander William T. Riker was trapped on Tilonus IV and subjected to a procedure to attempt to extract strategic information from his mind. As a defense mechanism, his mind created a delusion in which he found himself shifting between being on the Enterprise-D and as a patient in the Tilonus Institute for Mental Disorders. During one instance in which he believed himself to be in this insitute, one of the guards explained his headache as being due to him struggling with an attendant and hitting his head on a door. (TNG: "Frame of Mind")
In 2371, Major Kira Nerys discovered a door blocking her path when trying to locate a power source on the Founders' homeworld, one composed of a metal her tricorder could not identify. Odo found this odd, as his people had no need for doors, as they preferred not to take humanoid form. The two wondered who could be using it. Upon further investigation, they discovered that it possessed a locking mechanism designed apparently not to keep others out but to keep whatever was inside in. Odo then used his shapeshifting abilities to disable the lock. (DS9: "The Search, Part II")
Although the turbolift doors were easily operated by a members of the on set special effects department crew using a pulley (similar to common drapery pulleys), there were occasional accidents. In Star Trek: The Motion Picture, the victims of such incidents included William Shatner and Majel Barrett, both of whom were caught between doors that were closing too quickly. At times, the doors were also extraneously noisy. (The Making of Star Trek: The Motion Picture, pp. 160-161 & 183)
Destroying the Enterprise bridge for Star Trek III: The Search for Spock proved difficult because of the turbolift doors on the set, as they had been made from fiberglass. "And you don't blow up fiberglass with people nearby," noted special effects supervisor Bob Dawson. He solved this issue by reconstructing the doors with balsa wood. Dawson also fitted the explosives required for the destruction behind the doors. (The Making of the Trek Films, 3rd ed., p. 56; Trek: The Unauthorized Story of the Movies, p. 97)
According to Bill Wistrom, Douglas Grindstaff created the sound of the doors in Star Trek: The Original Series by squeeking with his tennis shoe. ("Select Historical Data Year Six - Timeless Sound Effects", TNG Season 6 DVD special feature)
In "The First Duty", Wesley Crusher's quarters at Starfleet Academy have an old-fashioned door with a handle. In a commentary for the episode available on TNG Season 5 Blu-ray, episode writers Ronald D. Moore and Naren Shankar relate that Rick Berman freaked upon seeing this door, stating that there were no doors like this in the 24th century. A beeping sound effect was added in post to make the door more futuristic, though it was only used once. Additionally, an internal backstory was created that this was an ancient wing of the academy that hadn't been renovated.