.com represents the word commerical and is the most widely used extension in the world. most businesses perfer a .com domain name because it is a highly recognized symbol for having a business presence on the internet.
.net respresents the word network and is most commonly used by internet service providers, web hosting companies or other businesses that are directly involved in the infrastructure of the internet.
.org represents the word organization, and is primarilly used by non profit groups or trade associations. it is also used by non-profit ministries and is very widely known and accepted.