(written from a Production point of view)
Syndication is the occurrence of local television stations purchasing individual television shows outside of a network context. Some stations carry network programming at peak hours, such as prime-time and weekends, and air syndicated series at other times. Others thrive solely on syndicated shows. Star Trek: The Next Generation and Star Trek: Deep Space Nine were both syndicated, and actually the first Star Trek shows to be released directly through syndication, instead of premiering on one of the three national broadcasters, constituting an innovative industry novelty at the time when The Next Generation was launched. (William Shatner Presents: Chaos on the Bridge)
The original Star Trek was aired on NBC, one of the three traditional national broadcasters, where it was canceled after only three seasons. The show experienced a strong revival in the 1970s when it became syndicated across America, including on local "UHF" channels in addition to being shown on major networks. In syndication, Star Trek was often aired late at night (after 9 or 10PM) or on weekends. Due to the different time allotments for syndicated programs, nearly all Star Trek episodes were cut of certain scenes to allow for running in their syndicated forms. This led to a popularity in "uncut" episodes which were available on VHS tapes (and later DVDs) or shown in their entirety as part of special programing, such as the 20th anniversary of Star Trek in 1987.
Syndicated Star Trek programing continued well into the 1990s and early 2000s; however, some later showings presented the episodes in their entire form, without cuts, or cut the episodes in different ways. This was most evident on the Sci-Fi Channel, which began showing Star Trek in the mid 2000s, which either cut additional scenes from those in syndication or showed the episodes in their entire form.
With the rise of digital streaming, syndicated Star Trek episodes have become a rarity and are even sought out for their uniqueness as collector's items.
The producers of Star Trek: The Next Generation originally sought a lower-pressure syndicated environment. When the owner of Star Trek, Paramount Pictures, launched its own network, UPN in 1995, Star Trek: Voyager was the flagship series. UPN was intended as a de facto fifth national broadcaster, after NBC, ABC, CBS and the relative newcomer Fox Television.
Voyager ended in 2001 and was immediately replaced with Star Trek: Enterprise, which was canceled after only four seasons as UPN attempted to change its image. Like the original Trek, Enterprise was nearly canceled a year earlier than it was, but was spared and moved to Friday night instead, where its ratings fell, leading to cancellation.