United Kingdom extensions ...
.gov.uk is a national or local government site eg www.opsi.gov.uk the Office of Public Sector Information
.co.uk is a company site eg. http://www.slc.co.uk/ the student loans company
.ac.uk is an academic site eg. http://www.soton.ac.uk/ the University of Southampton
.org.uk is an organisation, often a charity eg. http://www.nationaltrust.org.uk/ the National Trust
.ltd.uk is a registered company eg http://www.evans.ltd.uk/ the women's fashion outlet
There will be other domain extensions produced in the future. A recent introduction is .me.uk which is presumably meant for individuals.
However you must note that only some of these extensions can be relied upon. Only .gov, .ac and .ltd are strictly controlled. Anyone can set up a website with a .co or .org extension. In summary, you can use the domain name extensions as good CLUES (which are watertight for .ac.uk and .gov.uk) but be very careful.
Now let's look at United States extensions ...
