From Marty Becker's book "Meow Wow":

Most cats have five toes on their front paws, but only four of them hit the ground.

The fifth toe is called a dewclaw; it is found on the inside of the front paw.

The dewclaw is the equivalent of our thumb, and it's used for grasping prey and climbing trees.

A normal feline back paw, by the way, has four toes that are all called into service when walking.

Any number of toes over the norm (usually an extra one or two, but occasionally as many as three or four) makes a cat polydactyl, which means "many fingers."

Polydactylism is a dominant genetic trait, which means just one polydactyl parent is enough to make a litter of polydactyl kittens.

-- Robin Palmer