When choosing a domain to represent you, your company or your products, some important considerations come into play.

Short, Catchy, Memorable, and Brandable
You may want to consider a one word name like Google or Yahoo. If so, it is best to keep it short and one word. It's a sure bet you'll want to own a .com, as .com is still the most popular and best choice if the name you want is available. Many people will assume a name is .com, type it into their browser and expect to find your web site.

Company Name Equals Domain Name
Perhaps you already have a company name and want a domain name to match. Buy the .com if it is available, if not consider adding "co", "corp", "inc" to the end as people will understand the addition and in over time remember your name with the addition. Or consider the .net or .biz.

Product Focused
Your focus may be on having your product or service be your domain name. Or perhaps you may want a separate domain name from your company to launch or promote a new product

Keyword Centric
If you want to rank highly for a keyword or keyword phrase, and you are not in a highly competitive niche, you may want to choose a name with keywords in the domain.