Domain tells you what type
The grand-daddy of domain names, these domains are usually the most valuable in an international setting. If your product or service is available in many parts of the world, or you have an international trademark, a .com domain name is a must. Another advantage is that some web browsers add the "www" and ".com" parts for you, so if you type in "mycompany", it may come up with "www.mycompany.com". Also, if someone knows the name of your company but doesn't know your web site address, they may guess, and the first thing they will try is "www.yourcompany.com". A disadvantage of .com domains is that because they are so widespread and popular, many of the nicest sounding domain names have already been registered.
Kind of the "poor relation" of the .com domain name, these domains were originally designed to be used by internet companies. Examples of the correct use of these are virgin.net and earthlink.net. However, these conventions have been widely ignored for many years, and in most cases, someone will register a .net domain if the .com alternative has already been taken, or sometimes they'll register both (assuming they're both available), to stop someone else from running a similarly named web site.
A relatively new domain type, and therefore with many more variations currently available. One reason this domain was brought into being was to deal with the problem of a shortage of domain names. Another was that the .com domain, originally meant to indicate a commercial web site, has widened its appeal, and so a domain meant especially for business users was devised. However, you'll want to think carefully about whether the "biz" is suitable for your line of business.
Introduced at around the same time as .biz, this domain was intended for informational sites, rather than sites which sell or have other commercial purposes. This domain is unusual in that there are four letters after the final dot, whereas nearly all other domain names have either two or three letters. This can occasionally result in email addresses being rejected.
Originally intended for not-for-profit organisations, this domain type has been diluted to the point where many commercial organisations have registered it, sometimes simply as a protection against similarly named companies. However, it is expected that in the future, the not-for-profit rules may be enforced more strictly. Therefore, it is not recommended that you register a .org domain unless your organisation is a non-profit making one.
Mobi is the first and only top level domain name dedicated to delivering the Internet to mobile devices. It is designed to guide mobile users to made-for-mobile Internet content and services that can be accessed with confidence.The .mobi Registry is backed by the most prominent mobile and Internet players in the world - the very same companies (e.g. Nokia, Ericsson, Microsoft, and Google) who have delivered the promise of today's information society.