Star Trek: Deep Space Nine was the third Star Trek television series. Its first season was aired in 1993, overlapping the last season of Star Trek: The Next Generation. DS9 was on air for a total of seven seasons, and comprised of 176 episodes in all.
Deep Space Nine was the only Star Trek series to take place on a space station instead of a starship. The series was the first to have nearly twenty steady guest stars, some of whom's characters had significance to the storyline. New alien races were introduced in DS9 as well, including the Trill, Changeling, Vorta, and Jem'Hadar. Lesser known races like the Ferengi, Cardassian, Bajoran, and Breen from The Next Generation were thrown into the limelight, providing viewers with new insight on them.
DS9 was different from other series, because it was the only one to have a continually developing storyline from its beginning, to its end. This had been the intention of creators Rick Berman and Michael Piller, giving the writers an opportunity to create episodes which flow from one into the next. DS9 earned the nickname the "Soap Opera Star Trek." This approach was designed to intrigue viewers into watching the new episode each week, but most viewers stopped watching the show for that reason. For reasons I can't comprehend, Star Trek: Deep Space Nine became the least popular Star Trek series of the five.
The DS9 saga ended in 1999, along with the beginning of Star Trek: Voyager's fourth season.