PVP is a common acronym for Player Versus Player combat. PVP is permitted at MinecraftOnline, as long as it is not taken to excess and does not degenerate into bullying.
Any attack that damages another player is regarded as PVP. When one player attacks another, a message will be spawned saying: "[offender] started a fight with [victim]!". When you open another player's profile on the control panel, you can see their kills and deaths in PVP.
Server-wide death notification messages are sent when one player kills another in PVP.
Since 20 Feb 2012, it is possible to collect Player Heads by PVP combat.
Killing a player in PVP will bring you a visit from the Steve police.