(written from a Production point of view)
Established by Paramount President Network Television, Lucie Salhany (who had been responsible for the successful launch of Star Trek: The Next Generation in direct syndication, an industry first at the time), UPN launched on 16 January 1995 with the two-hour series premiere of Voyager, "Caretaker". They broadcast a small number of other original shows, but the initial programming lineup was mostly syndicated shows drawn from the vast body of programs available to Paramount Pictures. Voyager was the only original program from the network's launch to last into its second season. When Voyager ended its television run in 2001, Enterprise replaced it.
The idea of a Paramount Pictures television network was not new to the 1990s; in the late 1940s and early 1950s Paramount was partner in the failed DuMont Television Network, and in 1977 there were plans for a Paramount Television Service to compete as a 4th network alongside CBS, NBC and ABC. Star Trek: Phase II was intended to be one of the lead shows of the new network. The plans fell apart though, and Phase II ultimately became Star Trek: The Motion Picture. Eventually the market niche of "4th network" would be filled by FOX in the 1980s, ironically under the auspices of the same CEO who had tried to do so for Paramount earlier, Barry Diller. Even more ironic was that Diller had enticed Salhany to leave Paramount in 1991 to head his Fox Broadcasting Company, but she returned to Paramount to succeed for the studio where her former boss had failed, back in 1977. 
In 2000, the network officially changed its name to just the initials "UPN". In the early 2000s the network began to change its focus by targeting an African-American audience and produced urban-themed situation comedies with African-American casts, as well as professional wrestling and reality shows. This change in targeted demographics and programming is considered to be highly influential in the cancellation of Enterprise after only four seasons of a projected seven season run.
UPN, then owned by the CBS Corporation, the former Viacom, ceased to exist on 15 September 2006 when it combined with Time Warner's "The WB" network to become a new network called "The CW". With no Trek series in production since 2005, Star Trek is not a part of this new network's schedule.