Generally, a domain name should be chosen which reflects your company or your web site's content, although this is simply to make the site address more memorable to visitors.
Most companies with a known brand name which is important to their business will choose to register their company name with the appropriate root domain extension (.com, .org, etc..).
Choosing the right root domain is a bit more difficult, and really depends on how you wish your company to be perceived. See the section on 'What do the different domains mean' to help with this.