From the publisher:
Vermin is a d20 game set in the streets of a dark fantasy city. Unlike most fantasy games, this one's set at dirt level--you play the tiny beasts, the dirty vermin of the city. Players can choose between Rats, Mice, Sparrows, Squirrels, Toads, Roaches, and Lizards.
These vermin are a bit different from the ones you and I know--they have human level intelligence, and tool-making ability. That said, they're smart enough not to let humans (the biggest enemy of all) find out about all this. (Hmm... adventure hook already.)
[...] Unless specified, this game follows the same rules in [ Dungeons & Dragons (3rd Edition) Player's Handbook ], especially regarding character creation, experience, and combat. Feats that aren't explicitly explained follow the same rules as in the PHB and the same goes for skills.