A hairball is a small collection of hair or fur formed in the stomach of animals, and uncommonly in humans, that is occasionally vomited up when it becomes too big. Hairballs are formed from hair or fur that is delivered into stomach orally. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as swallowed food. Animals with hairballs are sometimes mistaken as having other conditions of the stomach such as lymphosarcoma, tuberculosis, and tumor of the spleen. (source)