Kong makes many different kinds, but here are a couple of the basics. It's basically something that you can put a paste, or treat in to occupy them
Rubber Kong Dog Toys - Toys - Dog - PetSmart KONG Stuff-A-Ball Dog Toys - Toys - Dog - PetSmart Kong® Large Dental Stick for Dogs - Toys - Dog - PetSmart
I have a couple of the ones in the first link and the third link. Some people buy the Kong paste to go with it, but it's probably better just to put yogurt or peanut butter in it instead.