.com usually designate commercial sites, .co.uk is used for UK sites, .tk are from a little island somewhere
DOT.TK 
The domains allow for localisation, specialisation and allow people to know roughly what sort of site they are going to and also mean more addresses can be made available for use (bit like dialling codes in England)
Here's a list of a few
http://www.norid.no/domenenavnbaser/domreg.html