According to the Council of European National Top-Level Domain Registries....(
http://www.centr.org)
Every country has its own registration rules, that's why there is such a diversity in registrations under the top-level and/or second-level. A top-level is the country code (like .uk), a second level is what stands before the top-level (e.g. the "co" in co.uk). The reason why some countries choose to register under (e.g.) co.uk or org.uk , is to give more people/legal unities the possibility to register the same name under the same country code. Let's assume that "Steve" is a well-known company in the UK. The company would probably have an interest in registering "steve.co.uk". However, some other, private, person called Steve (e.g. - you) might be interested in having his/your own domain name too (even though not running a company or business) - then you have the possibility to register under steve.me.uk. Also an organisation called "Steve" could have the possibility to register steve.org.uk - and so on. Another reason for registering under second levels like co.xx or com.xx might be that the country doesn't want someone else to register "com.xx" - and then start to do own registrations on the third-level . This is, however, mostly only a secondary reason. There are some countries (e.g. Poland) that are allowing registrations both under the top-level AND the second-level - for the same reasons. This means that both steve.pl and steve.com.pl would be possible registrations. There is often a small price diversity between these names, as a registration under the top-level is more attractive (and more expensive).