Star Trek is a Science Fiction series that was created by Gene Roddenberry in the mid sixties.
Its current canon consists of 6 series, totaling 28 seasons (a total of 704 one-hour live action episodes and 22 half-hour animated episodes) and 10 movies (with an 11th in the works.) It was also the basis for many hundreds of novels, technical books, licensed products and toys.
Star Trek (1966-1969, or 2267-2268 in universe), sometimes called TOS (The Original Series) is often considered the best of the existing series due to its iconic characters and villains, as well as stories written by established science fiction authors of its day. It was set aboard the USS Enterprise as the characters explored the "frontiers" of known space in a sort of analogy to the United States' own days of westward exploration as well as engaged in some Cold-War-era military confrontations.
Star Trek: The Animated Series
Star Trek: The Next Generation (1987-1994, or 2354-2370 in universe) was set aboard the USS Enterprise-D. It has featured the highest ratings of any Star Trek series thus far.
Star Trek: Deep Space Nine (1993-1999 or 2369-2375 in universe) was set aboard a (Bajoran owned, Federation administered) space station near the newly discovered Bajoran Wormhole, as opposed to the traditional starship setting. The show often meets with mixed reactions, as some prefer the initial concept of the show being focused on the Starfleet personnel and their interactions with the recently liberated and impoverished Bajoran people, and some prefer the later Dominion War focus of the show.
Star Trek: Voyager (1995-2001, or 2371-2377 in universe) was set aboard the USS Voyager, which was stranded in the distant Delta Quadrant and trying to make its way back to Federation space with a crew comprised of both Starfleet and Maquis personnel. It is often regarded as a terrible show because some see it as having poor story ideas and poor consistency despite a promising premise.
Star Trek: Enterprise (2001-2005, or 2151-2154 in universe) Initially called Enterprise, this prequel series was set before the birth of the Federation. The show was not well received, and most complaints about the show seemed to mirror those levelled at Voyager. It was the first Star Trek series since The Original Series not to run for at least seven seasons.
In the Versus Community
Within the culture of internet versus debates, Star Trek is one of the old standbys due to its wide recognition and plethora of available information.