Another high level domain type (like .com, .net and .org, etc.) for use on the World Wide Web, primarily used for British websites.
Also a good second choice when choosing domain names; the original .com is the most sought after by web developers etc. for its instant recognisability but many feel that .co.uk carries a certain amount of prestige value also.
This is possibly because the UK as a country arguably attracts the second largest amount of worldwide attention after the United States so being tied to the British identity in this way carries a certain amount of kudos. Also possible, a .co.uk site, as with a .com, is almost certain to be in English, the most widely used language on the web, thus attracting more visitors.
This is most often seen with British sites
who'll often choose a .co.uk
instead of or as well as a .com