(written from a Production point of view)
Established by Paramount President Network Television CEO Lucie Salhany and Paramount Television colleague CEO Kerry McCluggage, UPN launched on 16 January 1995 with the two-hour series premiere of Voyager, "Caretaker". Voyager was slated right from the start 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 of Paramount Network Television Garry Hart. Hart 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.
Once UPN was in place, the network became the primary outlet for all (live-action) media news regarding Star Trek, and several Star Trek specials aired on the network while Star Trek was in production in that time period. Previously, the Paramount Television produced and syndicated current media affairs show Entertainment Tonight held that position. None of the specials aired by either, though, found their way onto later released home media formats, despite being franchise property.
The idea of a Paramount Pictures-owned national 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 fourth network alongside CBS, NBC, and ABC. Star Trek: Phase II was intended to be the flagship of the new network. The plans fell apart though, and Phase II ultimately became Star Trek: The Motion Picture. Eventually the market niche of a "fourth national network" was filled by FOX in the 1980s, ironically under the auspices of the same CEO who had tried to do so for Paramount in 1977, 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 have been 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, demographic interpretation, and air time scheduling were concerned. In this respect, it had disturbing similarities with what had occurred between Star Trek: The Original Series and NBC in the 1960s. Executive Producer Rick Berman 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." (Star Trek: Enterprise - In a Time of War) Another example was, according to Brannon Braga, their decree that if the series was to be renewed for a fourth season – the network was actually already of a mind not to do so – the producers would get rid of Scott Bakula as Jonathan Archer. Berman fought this demand 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 final 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". (Before Her Time: Decommissioning Enterprise)
In February 2005, however, top management in the person of Viacom co-head Les Moonves – not known for his affinity for science fiction in general and Star Trek in particular  – cancelled Enterprise definitely, thereby ending Star Trek prime for the time being and ultimately UPN all together in hindsight. (In Conversation: Writing Star Trek: Enterprise, )
UPN, then owned by the CBS Corporation, the former Viacom, ceased to exist on 15 September 2006 when Moonves combined it with Time Warner's "The WB" (Warner Bros., and Moonves' former employer) network to become a new network called "The CW", which became an equally co-owned property by CBS and Warner Bros.