From BrWiki
DM or Dungeon Master is a name used in D&D to refer to the person who organizes and runs a role-playing game. Other (polite) terms include Games Master, Story teller, and the like.
Being a DM takes a lot of time - while players can just turn up, most DMs take hours preparing for gameplay in order to run a game as best they can, and frequently shell out on game products on behalf of the group. Players who want to help a DM should consider sharing in building the game world - rare is the DM who will frown upon well made maps or art, character histories that leave a number of adventure hooks, write-ups of villages, inns and the like that can be used in gameplay.
In short be nice to your DM, they do a lot of work to make your game fun and enjoyable experience.
