Hairballs, also called trichobezoar or fur balls, develop when a cat grooms herself with her tongue and ingests the hair. This hair can form into a mat or a ball within the digestive system. The presence of this foreign material can lead to gastrointestinal symptoms, especially if it obstructs the pathway of food to the stomach. As it makes its way further into your cat's system, it can cause constipation. Frequent grooming, hairball products and special hairball treats can help reduce the incidence of hairballs.
For more information, please read Dealing With Hairballs.