I'm not really sure how you can not find taht with the seach but o well I'll explain it anyway.
It depends on what faction.
For Balthazar faction, you have to go to the balthazar priests (i think that is their name) in the PvP outposts. They let you trade faction to unlock things for PvP only characters.
For luxon/kurzic faction, you spend it at the main luxon town (for luxon) or the main kurzic town (for kurzic). You can trade the faction for things like materials or give it to certain npc's (forgot what they are called, but they aren't too hard to find). The point of giving it to the npc's is to
1) work towards a title that depends on how much faction u donate to the npc's and
2) try to claim a city for your guild/alliance
I think taht sums it all up.