I don't know much about the technical reasons for domain extensions but think that some organistation is better than none. For the most part it's a country extension. The key ones I think are also easily memorable .co. .edu.
I believe there are hosting services out there that allow you to have .anyextension you like but I don't know much about them.
... Ford trucks, why can't it be www.ford.trucks or ford.auto
The ending part of a domain (.com, .de, .uk etc) is like a routing code - it tells the name server where to go to look for more details. This wouldn't work if you just had any random words separated by dots.
It would be similar to having random 12-digits phone numbers worldwide - you wouldn't have a clue where the number you're calling was, and more importantly the phone system would have to query some global database of all numbers to even start routing it. At present, the phone system can start routing with an area or country code, the domain name system can start routing based on .com, .net, .uk etc.
It would work even better if just country domains (.us etc) had been used to stop the system being dominated by the non-geographic .com (but think of .com as similar to a non-geographic or toll-free number - just as popular online as in the real world).
