At the edge of the Alpha quadrant lies a world that has survived through the occupation of the Cardassians, this world is Bajor.  After the occupation ended, Bajor and the Federation took over a 

Also, in second command of the space station, Major Kira Nerys (Nana Visitor), promoted to Colonel, a Bajoran woman who suffered through the occupation and now hates almost every Cardassian, including Elim Garak, a so-called Cardassian tailor who used to be an agent in the Cardassian secret service, most people think he is a spy, but there's no proof.  Odo (Rene Auber Jonois) is a shape shifter, a changeling, a metamorphic who is the chief security officer on board the station.  Jadzia Dax (Terry Farrell) is a trill, who is joined to the Dax symbiont.  Dr. Julian Bashir (Alexander Siddig) is the chief medical officer on the station, Miles O'Brien (Colm Meaney) is the chief engineer, Quark (Armin Shimerman) is the Ferengi bar tender, Jake Sisko (Cirroc Lofton) is the son of Captain Sisko.

Later in the series, Lieutenant Commander Worf (Michael Dorn) comes on to Deep Space Nine in the episode called "'The Way Of The Warrior I&II".  Worf, also known from Star Trek : The Next Generation, is the only Klingon in Starfleet, he married Jadzia Dax who later died but the Dax symbiont survived and was put into Ezri Dax (Nicole deBoer).

The worm hole to the Delta quadrant was good news, because it was the gateway to a whole new quadrant that hasn't been explored.  But soon it turned into bad news, when Benjamin Sisko made first contact with The Dominion.  The Dominion took over most of the Gamma quadrant, to bring peace and order.  The leaders of the Dominion are the Founders, they are Odo's people.  Soon after, the war with the Dominion started, when they joined Cardassia to take control of the Alpha quadrant and that means they would control half the galaxy...