The Lord of the Rings Online's PvP system is unique. In the main PvE game, you can't play as an evil soldier who serves Sauron, the Witch-king, or Saruman; you can only play as a member of the Free Peoples of Middle-earth. Obviously, when every one's on the same side, coming up with a good PvP scenario becomes problematic.
Turbine solved the problem by letting players log into pre-made but customizable evil characters whose functions are limited to PvP. Other players take their normal good-guy characters into the battlefield to fight the "Monster Players." It works pretty well. The PvP in LotRO is surprisingly deep for something that at present occurs in only one zone.
Deep means complex, though; there's a lot to learn. That's why fans at the official LotRO forums decided to put together a comprehensive strategy guide to playing a Monster Character. It's a great resource, and the community is still updating it with new strategies.