.uk is the Internet country code top-level domain (ccTLD) for the United Kingdom. As of March 2012, it is the fourth most popular top-level domain worldwide (after .com, .de and .net), with over 10 million registrations.
The use of .uk rather than .gb (which is the UK's ISO 3166 country code) is due to its pre-existing use in the (now obsolete) JANET Name Registration Scheme. .uk was made available in DNS to simplify the translation to and from these addresses. There were plans for eventual transition to .gb, but this never occurred; the use of .uk is now entrenched, and it is impossible to register a .gb domain.
New registrations directly under .uk are generally not permitted. Instead, registrations are made under a selection of second-level domains.
.uk uses OpenDNSSEC.
Read more about .uk: History, Second-level Domains, Allocation of Domain Names, Sch.uk