Let's say you own number1.com and number2.com. If you want them both to have the same website, you can make number2.com point/forward to number1.com, that way when someone goes to number2.com it points him to number1.com
Also, when I first got a domain I used domain (aka URL) forwarding to forward to my ISP hosting. For example www.mydomain.com would forward http://worldnet.att.net/~username. Also nobody would ever know. It saves money. A good company that does this is dotster. They also give you space for your own email at your domain w/ url forwarding.