.com represents companies or for profit organizations, it is also the most common extension for domain names. Most personal web sites also use this extension.
.net generally represents Internet Service Providers.
.org represents non-profit organizations or groups.
.info is the first of the new top level domains. It is designed for general use and is expected to become as popular as .com.
.biz is also a new top level domain. It is designed for general business use and is expected to become as popular as .com.
Though these are general guidelines, you may use any of these extensions when registering a domain name. If you want to be make sure no one else can benefit from your domain name, it's a good idea to register all three variations of your domain name.