Cats and kittens will self-groom several times daily. In fact, over 80% of a cat's time is spent in pursuit of cleanliness, not surprising when you know that they have over 60,000 hairs per square inch on their stomach's alone.
Their tongues have tiny structures that are meant to grab and pull loose hairs from the coat which are subsequently swallowed.
The unfortunate consequence of this grooming (especially in long-haired cats and during the Spring and Summer shedding time) is an accumulation of hair in the stomach. Generally, it passes through the digestive tract unhindered or is vomited up to get rid of it. However, in some instances it can block intestines or cause other digestive problems.
Not to mention that it is a fairly unpleasant thing to find on the floor or your bed.
Brush your animals frequently to collect as much loose hair as possible, and give your cat a tasty hairball product daily. Put a dollop of remedy on the bottom of one of his front feet. His very cleanliness will require him to immediately clean it off, solving the problem of ingestion.
Cats naturally eat grass when their stomach is upset. Provide them with green grass or roughage to eat to help them vomit the hairballs that collect. If they can't go outside, purchase a container of "cat grass" that will grow in their favorite window sill.