Monster Play is a PvP (Player vs Player) element included in LOTRO that enables you to join the forces of evil and battle high-level gamer characters. When you reach level 10 with your basic character, you can access Monster Play using the Fell Scrying Pool. These are sample locations of these portals:
Michel Delving (South-Western part of the Shire)
When you activate a Fell Scrying Pool, you'll be prompted to choose your evil alter-ego. You can decide from Uruk Warleader, Uruk Blackarrow, Orc Reaver, Spider Weaver, and Warg Stalker. These are evil counterparts (more or less) of Captain, Hunter, Champion, Burglar, and a Guardian.
After choosing your character, you'll be teleported to LOTRO's PvP area - Ettenmoors. You are automatically granted level 50 when entering. The point of this part of the game is to battle players who enter this area with their basic characters (they can do that only after reaching level 40). You can't advance your level as a monster, but you can get new abilities. To do that, you have to accumulate Destiny points and then spend them at the local trainer's.
Apart from PvP battling, playing as a monster is quite similar to normal course of gameplay in LOTRO. The quests are a bit simpler (most of them are about killing a number of certain enemies). Keep in mind that despite you're level 50, all the monsters here are not much weaker and you if you want to stand up to a drake or a dragon.. you'd better find yourself a fellowship.