CO-OP server = the opposite of PvP... you can't kill anyone else anywhere except in duels - you can go to any realm, kill any monster etc. but there would be no battles at all. Basically it's Mythic's apology for the PvP servers.
afaik the co-op server is a server where all keeps are initially claimed by social mob groups, with a uber boss(lord).
The idea is that instead of fighting eachother, you join ranks and fight the mobs. They drop new seals for special stuff. And once in a while a zerg of mobs runs over a keep, kills all (or not if well defended) and claims it until new forces gather and slay ehm again.