World PvP: Fighting someone 1v1, 2v2 etc in a random area where both of you are evenly match and have a chance for both sides to win.
Ganking: Killing someone 2+ levels lower than you.
Griefing: Ganging up on someone (2v1, 3v1 etc)/corspe camping
PVEers do not think that killing anyone in the world is ganking.
They think that an 85 trolling the lower zones and killing is ganking and griefing. A PVEer's take on it is basically that it's someone completely immature and so bad that they have to resort to killing lowbies for the simple reason that they have no chance against anyone of equal level.
This is further evidenced by the fact that in expansions past, they would "camp" places like Redridge and X-roads, and "claim" they wanted to incite world pvp, yet when opposition arrived, they would high tail it out of the zone as soon as they got killed.
That pretty much sums it up right there - and I dunno about PVP servers, since I refuse to roll on one, but on my server I spent most of Wrath with my 80 PVP-geared rogue parked in Crossroads. I didn't even HAVE to kill them, most of the time, all I had to do was log him in, and as soon as they spotted a geared rogue they ran like hell. 'Trying to incite world PVP', my ass. There is no PVP if the 'P' runs away as soon as the 'VP' shows up.