Cats routinely develop hairballs due to their self grooming. Since hair is not digestible, it accumulates in their digestive tract. When the accumulation becomes large enough it can be an irritant. A cat will then vomit up the clump of hair AKA, hairball. If, for some reason, the hairball cannot be excreted (because the cat is grooming excessively and the clump becomes too large) it can cause a blockage in the digestive tract. This can be fatal to the feline. The technical term for the hairball is trichobezoar.

A bezoar (a mass found trapped in the gastrointestinal system) formed from the ingestion of hair. Trichobezoars are often associated with trichotillomania (compulsive hair pulling). Trichobezoars are rare but can be fatal if undetected.

More Info: en.wikipedia.org