(written from a Production point of view)
Established by Paramount President Network Television, Lucie Salhany, UPN launched on 16 January 1995 with the two-hour series premiere of Voyager, "Caretaker". Voyager was right from the bat slated to serve as the first flagship of the new network. For her efforts, Salhany became the second highest CEO of the new network, only answerable to newly appointed President Paramount Network Television Garry Hart, who himself was the direct successor of John S. Pike, with whom Salhany had become directly responsible for the successful launch of Star Trek: The Next Generation in direct syndication previously, an industry first at that time. 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.
Yet, with the 2013-2014 releases of the Enterprise Blu-ray sets, former Enterprise staffers, from producers to performers, have come forward with tales which also pointed at studio politics detrimental to Star Trek in general, and serious mismanagement of Enterprise in particular, especially where ratings and demographics interpretation, as well as air time scheduling were concerned. In this respect, it had disturbing similarities with what had befallen between Star Trek: The Original Series and NBC back in the 1960s. Executive Producer Rick Berman has divulged that the relationship between UPN and Star Trek, which had been a warm one during the production of Voyager had taken a turn for the worse, "Our relationship with the network was distant. And it wasn't embracing and warm and...a sense of working together that had existed in all the years before." (ENT Season 3 Blu-ray-special feature, "In a Time of War") Another example was, according to Brannon Braga, their decree, if the series was to be renewed for a fourth season – the network actually already of a mind not to do so – , to get rid off Scott Bakula as Jonathan Archer, which Berman fought tooth and nail, successfully as it turned out (though he had not been able to resist their decree to add "Star Trek" to the series title which was originally just Enterprise, explicitly intended as such). That the series was renewed for a last season, was in no small part due to the fact that strong backing was received from an unexpected corner; Scott Bakula has unequivocally cited Garry Hart, a Star Trek supporter, who had just been promoted to another position within the conglomerate, as the driving force behind the renewal, thereby thwarting the cancellation intents of his successor(s) at UPN for the time being, conceivably an instance of "studio politics". (ENT Season 4 Blu-ray-special feature, "Before Her Time: Decommissioning Enterprise")
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.