Can someone explain domain names to me?

Was just reading an article on the net about the new .eu domain, and it got me thinking.

So many countries have their own domains, which are typically displayed as;

.fr being for france
.nz being for new zealand
.it being for italy

etc etc etc

Now, I was always under the impression that .com was a US suffix. why then does .au, the domain for australia, work as being .com.au, the same being for the UK, why is it .co.uk as apposed to .com.uk or just .uk like all other euro countries? and why isn\'t australia\'s just .au?

just curious, i don\'t really understand any of this bable, and it doesnt really even matter, but I\'m just interested as to why they are different.

top level domains

http://www.howstuffworks.com/dns.htm