Lice are a type of wingless insect, which live on the skin of animals for their whole life and can only survive for a few days in the environment. Unlike many other parasites, lice are very host-specific, so will only infest specific species. Lice can either be chewing lice, which chew on the skin of their host, or biting lice, which suck blood, however the only louse which affects cats in the UK is a chewing louse.
Louse infestations are more common in elderly animals, especially cats with long hair, possibly due to their reduced ability to groom themselves. Thankfully, infestations are not common in cats in the UK, although they can occur.
The only cat louse in the UK is called Felicola subrostratus. An uncommon parasite of cats, Felicola is a chewing louse and feeds off skin calls.