There are different levels of domains in the domain name. The second and third levels are the most important. The third level is actually optional on some sites.
Top Level (TLD)— .com, .net, .org, .edu, etc.
Second Level— business, shopping, search
Third Level— www, www1, www2, keyword (optional)
The top-level domain (TLD) name is the extension, after the actual Web site name. In the 1990s, most top-level domain names were limited to .com, .net, .gov, .org, .mil, .edu, and international “country” designations.
.com refers to a commercial organization
.edu is for education institutions
.net refers to networking companies
.gov is reserved for government pages
.org is for organizations or nonprofits ...