Domains are secondary domain names added to your account that you can make appear to visitors as individual, full functioning domains. The only primary difference is that the secondary domain names will not have their own individual control panels.

Example: Let's assume you own 2 domain names (test.com and test2.com) and you'd like for each of them to include different content. The only problem is you're on a limited budget. If test.com is your primary account domain name and you'd like to use test2.com for a new site, you can add test2.com as a secondary domain name pointing to a subdirectory on your primary domain name, test.com. The secondary domain name, test2.com, can be added in a way that the visitor will be redirected to the subdirectory on your primary domain name without even knowing it. The URL in the browser will show up as "http://test2.com" instead of "http://test.com/subdirectory".