The N'Kai are an ancient and powerful alien species that has existed since before the Earth formed 4.5 billion years ago.  Currently, they are fighting a losing war against the Space-Spawn.  Another alien race is also involved in this war. 
Their war has lasted for many centuries, and they have positioned Sentinels on various worlds to watch for Space-Spawn invasions of those planets. The only known member of this species is Nokkar, who appeared in the episode "Sentinel".
N'Kai are basically "humanoid" in appearance with especially human-like faces, and they are extremely long-lived (Nokkar has been living on Easter Island since A.D. 400). They seem to have no trouble breathing the Earth's atmosphere.