From GuildWiki

A griefer is someone who seeks to anger and irritate other players. Griefers are most often found in unorganized PvP, where there are few lasting consequences for their actions.

In Guild Wars, there are several common forms:

• someone who quits a team shortly after starting a match or area, with no explanation.
• a griefer may kill themselves, which can score points for the opposing team and may waste Resurrection Signets.
• some griefer builds use healing and defensive skills to the exclusion of any offense, making a build that is nearly impossible to kill, and simply waiting for other players to quit in frustration.
• griefers may run and kite with no other purpose then drawing out the conclusion of a match there is no possibility of winning.