.com is for business websites (like Sega, Sears, etc.)   

.edu is for educational institutions or colleges (like Princeton, Vanderbilt, Belmont etc.)    

.org is for non-profit organizations (like Middle Tennessee RockHounds a group of rock collectors)

.net is for network. (like Comcast) This can be non-profit but often is a profit company that manages a netowrk.

.gov is for sites put out by the government (like the White House)   

There are a few more endings, but these are the most popular.

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