There is no hard and fast rules on using extension. Following are some common usage of extensions.
.com Stands for “commercial” and it is an unrestricted international domain name extension. Most companies choose a .com domain, because this domain represents e-business and is the most popular domain on the Internet. Due to its popularity, however, the number of available .com domains is decreasing drastically.
.net The .net domain is preferred by companies whose business is directly connected to the Internet, such as Internet Service Providers (ISPs) and web hosting and development companies. Some companies use the .net extensions for their intranet sites.
.org The .org domain is intended for use by non-profit organizations
.info Information. .info sites provide general information, and are intended for both companies and individuals
.biz stands for “Business Internet Zone” and is a restricted international domain name extension reserved for business entities. The .biz domain was introduced recently and as a new domain name extension it gives companies much wider choice of suitable domain names than the overcrowded .com domain
.us United States. You can only use this extension for sites hosted in united states. The name servers should also reside in United States
.name This extension is used for personal websites. .name sites allows individuals to have a section of the Web zoned solely for personal use. .name extensions are great for building your personal identity online.