.dungeon is a book about traveling through a virtual, fantasy world. One person takes on the role of the World, filling in the nooks and crannies with dungeons and adventures that they think up or borrow from books they already own. Everyone else is a Player and creates a Character to explore those nooks, discover their treasure, and interact with NPCs and other flesh-and-blood folks in the virtual world.
The thing that sets .dungeon apart from other books is the inspiration it takes from social games like Werewolf and Munchkin. The real world can (and will) affect the game you create. Each class has different powers based on physical things you do, or different parts of your life. The witch uses their tattoos to cast spells while the wizard uses their favorite book. The knight makes vows while the leader can offer gifts to the other players to aid them. There are 10 classes total to choose from, including the bard, the troll, and the artist.