dammit, quit whining and node the damn show!
The Plot, In Brief
DEEP SPACE NINE
DS9 is about a space station located next to a wormhole that leads to the other side of the galaxy. It was originally built by the Cardassians as Terak Nor, part of their operation to enslave the people of the planet Bajor. Eventually, the Bajorans managed to get rid of the Cardassians, but they couldn't take care of the station on their own, so they called in the United Federation of Planets (which includes the homo sapiens of Star Trek) to run the station for them.
Until the Federation showed up, nobody knew about the Bajoran wormhole. When it suddenly appeared, the Federation sent a ship into it, and met a crazy empire from the Gamma Quadrant called the Dominion. The first season of DS9 basically introduced the Dominion and the Bajorans: the actual Dominion War began in the second season, when DS9 fans got to meet the Dominion's slave soldiers, the Jem'Hadar.
Around this point, Star Trek: The Next Generation ended, so DS9 began flexing its muscles. For the third season, the station was given a way-cool starship with a cloaking device, called the USS Defiant. With their new ship, the DS9 crew were able to learn about the changelings in charge of the Dominion, called the Founders.
Then came the fourth season premiere, "The Way of the Warrior," which was one of the best Star Trek episodes ever. The Klingon Empire began plotting to invade Cardassia, which just had a coup of sorts and was being reorganized under a civilian administration. DS9 brought in everyone's favorite Klingon, Worf, and then ended up helping the Cardassians to fight off the Klingons. After the episode, Worf stayed on the series and became a regular character. Rock on.
The Cardassians went on to become part of the Dominion, so the Federation, Klingons, and Romulans teamed up (again) to fight the Dominion, Cardassians, Breen, and Vorta. This war took up most of the last four seasons, and ended when the Federation released a powerful biological weapon on the Founders, and the DS9 crew used their know-how to extort a peace treaty from the Dominion in exchange for the cure. By the end of the series, the entire galaxy was pretty much wasted... but that was what made the show fun.
- Cdr. (later Capt.) Benjamin Sisko (Avery Brooks): DS9's commanding officer, a deep-voiced fellow born in Louisiana. He has an army brat son, Jake Sisko, by his late wife, Jennifer Sisko, and he also has a recurring girlfriend named Kassidy Yates. Because he was the first person to go into the wormhole, the Bajorans call him "Emissary" and revere him as a spiritual figure.
- Maj. (later Col.) Kira Nerys (Nana Visitor): She heads up the Bajoran administration on DS9. She was one of the leaders of the Bajoran militia during the resistance, so she's very hard-assed and she complains about Bajor's squabbling parochial government a lot. She also, understandably, despises the Cardassians. She has an ongoing subtle flirtation with Odo which fanfic writers love to play with.
- Odo (Rene Auberjonois): DS9's security chief. He's a Founder, although he didn't know this until the middle of the series, as he was found floating in space and brought to a laboratory on Cardassian Bajor. Like the other Founders, he's a shapeshifter: in his ground state, he looks like a puddle of ooze. This makes him a handy plot device, although you don't see the effect that often because he doesn't like to shapeshift in front of other people.
- Lt. Julian Bashir (Alexander Siddig): DS9's medical officer... fresh out of medical school, but he became a pretty good doctor once he was in the field. Later in the series, we find out that he was born with a learning disability, and that his parents had him genetically enhanced.
- Lt. Jadzia Dax (Terry Farrell): DS9's science officer and resident hot chick. She's a Trill, so she has a several-hundred-year-old worm living inside her, which has previously been inside men. One of these men, Curzon Dax, was one of Sisko's friends back in the day, so Sisko often calls Dax "old man." She fell for Worf mid-series and eventually married him, but her character was then killed off and replaced by Ezri Dax. Those Trills are handy plot devices.
- Officer Miles O'Brien (Colm Meaney): DS9's chief engineer, who was brought over from TNG. He's not exactly as stereotypical as, say, Scotty, but he still likes darts, has a short temper, and spends a lot of time in the bar. He is married to Keiko O'Brien, and he has a daughter named Molly O'Brien.
- Quark (Armin Shimerman): Quark is a Ferengi, which basically tells you all you need to know about his character. He's ugly, greedy, sexist, and constantly getting himself into trouble, but the administration often finds uses for him. He runs the station's bar, which also has a casino and miniature holodecks where patrons can get their freak on. He has a brother named Rom, who has a son named Nog.
- Lt. Cmdr. Worf (Michael Dorn): Son of Mogh, brother of Kurn, bad-ass in general, only Klingon in Starfleet, likes prune juice, widower of Jadzia Dax.
- Ens. Ezri Dax (Nicole de Boer): She was brought in toward the end of the series to replace Jadzia: since she was the only Trill nearby when Jadzia died, she was the only host that could take the Dax symbiont.