What is a Sub-Domain?

A sub domain is one, which resides under your top-level domain name, but in many ways behaves as a "totally independent domain". 

You'll observe that many of the larger corporations use these, as they're somewhat more professional looking, and do a better job of creating an independent precedence for service or product lines, which appear as separate web entities.

Example:

You're a GM dealer with a site such as GM.com. 

You sell everything from Pontiac's to Cadillac's. 

To better organize your online presence, you could create sub domains for your various automotive lines. 

These would appear as:

http://pontiac.gm.com/

or 

http://cadillac.gm.com/. 

Also note that in most cases, the domain need not be called with the http:// or www protocol.    

pontiac.gm.com can be called exactly as it appears here.