From publisher blurb:
The great mass of the Rat Horde lays in wait deep under the earth, an immense society unknown to almost all, even the other deep races. Dark stories tell of demonic ratmen that steal children in the night and conduct eldritch rituals in their dank warrens.
The truth is more mundane, of course, the Rats are a hardy, secretive people, but no more dark and evil than the rest of the races that make up the world. A clever, resourceful people, fond of traps, tricks and tinkering, their talent at building and scheming is outmatched only by their curiosity and stench
The Rat is a Companion Class for Dungeon World - Companion classes are complimentary subclasses designed for use alongside standard playbooks. At character creation, simply pick a normal playbook (Fighter, Wizard, etc) and also select a Companion Class. Throughout character creation and play, you may select options from either sheet.