Peeling is generally used to refer to either a player or mob being removed from another player. There are two types of peels.
The first type of peel usually involves another player intercepting and gaining aggro from a given mob (in essence, "peeling" the mob off that player) who is attacking one of the more squishy members of the group. This role or action is most commonly filled by a Hunter in the raid environment where peeling mobs off an initial aggro target to a tank is a common step in the pulling of many boss encounters that have multiple mobs involved. A boss where you might see a peel implemented is Majordomo Executus. In a PvP environment this can be achieved with methods of crowd control such as stuns, slows and the death knight spell
The other kind of peel is known as a self peel whereby you remove the player or mob from yourself. In PvE an example of this is the priest spell or in a PvP environment the paladin spell , druids and warlocks .