(written from a Production point of view)
The National Broadcasting Company, now formally called NBC Universal Television or, simply, NBC, is a US television network, founded in 1939 (as part of the NBC radio network, founded in 1926), based in New York City's Rockefeller Center. As of May 2004, it became part of NBC Universal.
The original Star Trek aired on NBC from 1966 to 1969. The network attempted to cancel the series twice, in 1967 and in 1968, but renewed it for an additional season both year, due to letter-writing campaigns. The animated Star Trek also aired on NBC from 1973 through 1974, with re-runs airing during 1975.
The original series was first aired Thursday nights at 8:30, where it replaced the show Laredo. For its second season, it moved to Friday nights at the same time. After being renewed for the third season, NBC moved it into the 10pm timeslot in place of the recently canceled Actuality Specials.
No Star Trek series has aired on the national NBC network since, although several of the motion pictures have made their network debut on NBC, including Star Trek VI: The Undiscovered Country (in 1994) and Star Trek: Insurrection (in 2001, although the initial premiere was pushed back several weeks due to the events of September 11, 2001. Hence, it finally premiered in 2002, to coincide with the theatrical release of Nemesis).