A cat litter box, sometimes called a sandbox, cat box, litter tray, cat pan, or litter pan, is an indoor feces and urine collection box for cats, as well as rabbits, ferrets, miniature pigs, small dogs, and other pets that instinctively or through training will make use of such a repository. They are provided for pets that are permitted free roam of a home but who cannot or do not always go outside to excrete their metabolic waste. Buy one here! 

The rule of thumb is to add 3-4 inches of fresh cat litter after dumping out the old.