It's a TV show — actually, it's a lot more than that. The original series followed the interstellar adventures of James T. Kirk and the crew of an exploration vessel of a 23rd century galactic "United Federation of Planets" — the Starship Enterprise. It ran from 1966 to 1969 and was followed by five other tv series set in the same universe, ten feature films — not counting the reboot, of course, which brings it to eleven — books, comics, games, figurines... pretty much anything you can imagine. There was even a Star Trek theme park in Las Vegas, but it got shut down in 2008.
[ To his own great dismay. ]
Star Trek is noted for its influence on the world outside of science fiction. It has been cited as an inspiration for several technological inventions such as the cell phone. Moreover, the show is noted for its progressive civil rights stances — the original series included one of television's first multiracial casts, and the first televised inter-racial kiss. The science involved is relatively accurate as well, which is rare for sci-fi... especially if you take into account that the original series was created in the 60s.