grey knights are pretty much space marines, except they answer to the inquisition. their geneseed is from the emperor, and they recieve the best weapons/armor the imperium has to offer. their homebase is on titan, one of saturns moons.
all grey knights are psykers (i might be wrong).
after fighting a daemon with an ally, they must kill all witnesses, since no one can stand in the glory of Chaos and be unaffected other than Greyknights. No grey knight has ever turned on his brothers and joined Chaos.