.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 and other organizations

.org represents non-profit organizations or groups

.info stands for .Information and is generally for information type sites but not limited to that type

.biz domains are used for businesses sites

.us stands for United States and can be used for any type of site

.ws means 'website' and can be used for many different types of websites

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.