Simple turn-based eight-direction grid movement, where one of your adjacent squares contains your "sword" and you can rotate your sword around yourself one square per-turn. Kill (deterministic and 1hp) monsters by moving your sword on to all the monsters move after your move, and "complete" a room by killing all monsters.
There's quite an active community (at least there was when I was in to it) but I've rarely heard it mentioned outside of the community.
Edit: I'm happy to see some other references to DroD in this thread.