It's a game (if that isn't obvious :P). A massive multi-user game, obviously based on some sort of Tolkienish medieval fantasy setting. Levels (53), races (human, troll), and character classes/professions (mage, rogue) reflect some sort of Dungeons'n'Dragons style system, I suppose. And the heart of the game is obviously "hack'n'slash": you beat monsters (and other players), obtain better equipment (like weapons) for your character, gain experience (usually by beating the monsters), and advance in levels -- become better and tougher.
The other posts in this thread mention servers: so the game is obviously massive enough to have been distributed on several servers.
More or less similar games are, for example, Ultima Online and Everquest. The trendy term for this sort of games is MMORPG, or MMO*.
http://en.wikipedia.org/wiki/MMORPG
I have never played any of these particular games, so I might be wrong.
-Methem