Looks like some people disagree on what these terms actually mean.
This is how I know them as:
Griefer:
Someone that invades at low level with high level gear with the goal of killing new players/defenseless players. This does not include players that invade with regular gear, or people looking for a low level fight.
Ganker:
Someone that waits with 1 or more summoned phantoms to 'farm' invading phantoms through any means necessary. This doesn't include new phantoms that don't stand a chance otherwise and are learning the ropes of PvP.
Troll:
This one I'm unclear about. I think people use it to mean someone that either hosts or invades and uses questionable tactics. Some guys on youtube had a video that had him invading in the Painted World, and a guy in Smoughs w. MotC waiting for him on the bridge. The guy would buff with DMB and fight. This guy considered this a troll for some reason; I'm not sure why forcing someone to fight on a bridge makes someone a troll. He didn't have any advantage this guy didn't/couldn't do. So "troll" I don't know what it means.
Is this the general consensus from everyone else?