Just as no other city can exist at the same exact longitude and latitude as Colorado Springs, the user designation is a unique location licensed to assure that no two users exist at the same exact web address. A domain does not really have an equivalent in the postal world. Like these formal titles, the domain indicates what the user is. The .edu designation indicates an educational institution. Public Broadcasting uses .org.
Different countries have different domains as well.
For example, .jp is the domain for Japan and .uk is the domain for Great Britain. Accessing web sites from different countries can slow down the transmission time of these sites. Keep this in mind as you learn more about the web. If you are in a hurry, you may not want to plan to access very many over seas sites in one session. It is like finding Colorado Springs on the map.