An identification card was a document carried by a person for identification purposes.
In the 2020s on Earth, civilians used identification cards such as UHC cards and transit passes to avoid ending up in San Francisco's Sanctuary Districts. (DS9: "Past Tense, Part I")
In 23rd century Starfleet practice, identification cards were carried by Starfleet personnel. The ID card showed a person's service number and name, as well as his or her assignment and division.
Identification cards could be created aboard Starfleet starships, and they were sometimes necessary to gain access to certain areas, like a ship's shuttlebay. (TAS: "Mudd's Passion")
Federation security agents used ID cards to identify themselves in 2285. (Star Trek III: The Search for Spock)
After having traveled back in time to 1986 from 2286, Pavel Chekov's ID card was seized by the United States Navy and read aloud by an FBI agent. The agent did not believe the information on the card, including the fact that Chekov represented Starfleet and the United Federation of Planets. (Star Trek IV: The Voyage Home)
Gary Seven possessed several faked ID cards as part of his undercover mission in 1968. They included cards from the Central Intelligence Agency, National Security Agency, Federal Bureau of Investigation, and the New York City Police Department. (TOS: "Assignment: Earth")