Domain names are simply addresses to websites. For example www.stanford.edu is a domain name.
A subdomain is one way of organizing and separating content on your site. You’re already familiar with the concept of subdomains, even if you don’t know it. Consider Stanford’s public Web site at http://stanford.edu.
As you browse parts of that site, you’ll notice that the domain changes. When you’re looking at your department Web site, say the site for the Chemical Engineering department at https://cheme.stanford.edu, the URL is no longer just stanford.edu. Now the root of the url is cheme.stanford.edu, indicating that you’re on the part of the site that is dedicated to the Chemical Engineering department.
As you can see the domains serve two purposes: they help to organize the site from a technical perspective, but they also serve as indications to the users that they are in a new/different space. As you work on your site, you’re welcome to create as many subdomains as you like, and in each subdomain you can actually create a distinct, individual Web site.
If you possess a domain (i.e., profx.su.domains), you are able to create an almost infinite number of subdomains associated with it (i.e., bioe44.profx.su.domains, bioe102.profx.su.domains) . In practice, this allows a user to simultaneously install multiple web applications to each of their domain and subdomains using WordPress, Omeka, MediaWiki, and/or Drupal .