Quick question about D&D

[Nitro Vordex]

What exactly do you do as a DM?

Also, there's another game that is like D&D, but I can't think of the name of it.

Wolf something? Maybe? I was around when my brother played it with his friends.

[NPCMook]

DM: Dungeon Master, you are the narrator of your world, the voice of your characters, the villian of which your adventuring party must defeat!

There are several DnD type games along side Dungeons and Dragons, theres the Star Wars D20, Modern D20, Warhammer Fantasy RP, Warhammer 40K: Dark Heresy RP. There is infact even a World of Warcraft Tabletop RP... and I don't mean the board game

[Rust]

Yeah, as a DM, you tell the story and take the role of all the characters and monsters who your players encounter ; you make the story evolve according to your players' characters actions. The DM is usually also the one who keep an eye on the rules and get the last word on them to make sure everyone is having fun. For the most dedicated DM, they go as far as building up a whole story and world / universe setting of their own, as well as creating their own custom rules. This probably is as much hard work as having fun when you have lot of things to prepare, while the players actually just have to play, but when the 'job' is done well, this is a very rewarding and fulfilling role to take on.

As for the game you're talking about, the only one which comes to my mind is Werewolf, from the World of Darkness brand (including Mage, Vampire : The Masquerade, etc...), but it's not d20 system at all. Like NPCMook said there are plenty of d20 system RPG (while he's talking about WoW RPG, there is also a Diablo II one, for instance).