.COM - from the word commercial was intended for businesses but due to it's popularity its become the first place you should search for your domain name.
.ORG was originally meant for sites that represent organizations and non-profits. This is an open domain extension and any person is permitted to register.
The newly launched .CO is speculated to become popular and a runner-up alternative to .com.
.BIZ is one of the rarer on the list. It is intended for business use, but its popularity pales in comparison to .COM
Country code specific domain names contain two letters following a .co. For example the country code for Kenya is .CO.KE and for United Kingdom is .CO.UK.
Other popular extensions include .NET intended for network web sites, .ME often used for personal websites, and .INFO open for any use.