Sparring (Right-click on opponent, select Sparring - Spar) is a "friendly" fight between two opponents, to which both parties must consent to. As such, it is a relative "safe" PvP fight that can happen on PvE servers. No fighting or weapon skill is gained by Sparring.
The server issues a "Stop" message before either party is killed. The winner may not loot the loser, who should still be alive at the end of the spar.