There are two types of domain name extensions: Generic .net, com. org and country codes .us, .uk., .de, .jp.
In the generic group .com is simply the most valuable because its the first association that people think when it comes to domain names. Other reason is also .com was the first domain name that made .com business popular because it was one of the first extensions.
Then follow .net, .org, .biz and .info. These domain names have the same status as .com, but they are just no so valuable. There are also lots of discounts on .info domain names, because they just don`t sell so good.
.mobi - is a special domain name meant for mobile users.
.aero - for flying business
.museum
.travel and so on.
Country codes are intended for a specific territory.
I have written an article which covers all of them, I can`t remember all now
