What it is.

In D&D or Dungeons and Dragons, you create a character which you control. You said what you want to do, and the dice help determine the sucesses, with the Dungeon Master ultimatly weaving the world around you.
You could be a fighter, a cleric, a wizard, a ranger, a rouge, a monk, a barbarian, a bard, or something more exotic.
You could also play as various races such as Human, Elf, Dwarf, Caliban, Half-Elf, or something more exotic.