The protection tree is the paladin or warrior talent tree designed specifically for tanking. This involves building up one's defense and one's ability to receive more damage. It revolves around shield use, the righteous fury for paladins or the defensive stance for warriors.
The protection tree is the most PvE-oriented for a warrior, as warriors are normally called upon to tank.
The paladin's protection tree is also focused around tank abilities. Due to the Paladin's AoE Tanking style (via 
[Consecration]) it is also useful for taking numerous mobs at once in PvE, and "AoE Grinding".