For questions specifically about Star Trek: The Original Series which aired in 1966. Always use in conjunction with the [star-trek] tag. Users should clarify if they want to include or exclude the TOS-era films in the answers, since we have no separate tag for those.
Star Trek, now referred to as Star Trek: The Original Series, was the original television series created by Gene Roddenberry that spawned the extensive Star Trek franchise of television series, feature films, novels, comic books, and video games. The Original Series ran from 1966 to 1969 and aired in the US on CBS.
Set in the 23rd Century, the series follows the first three years of the five-year deep space mission of the Federation starship uss-enterprise. Under the command of Captain James T. Kirk, the Enterprise was one of several vessels sent by the Federation's Starfleet to explore unknown parts of the galaxy and make contact with new life. The series introduced the two primary antagonists of the Star Trek universe, the Klingon Empire and the Romulan Star Empire.
The show became a cult classic in broadcast syndication during the 1970s, leading to five additional television series, 11 theatrical films (and counting), and numerous books, games, and other products.