What do they mean by Top Level Domain?

Although there are many domain extensions now, prior to the creation of ICANN in 1998, there were only seven generic top-level domains. They are:

1. .com (commercial) administered by Verisign
   The most popular domain extension by far.
2. .org (organization) administered by Public Interest Registry
   Intended for use by non-profit organizations but open to anyone
3. .net (network) administered by Verisign
   Intended for use by domains pointing to a distributed network of computers
4. .int (international organizations) administered by Internet Assigned Numbers Authority
   Strictly limited to organizations, offices, and programs endorsed by a treaty between two or more nations, however, a few grandfathered domains do not meet these criteria.
5. .edu (education) administered by Educause (via Verisign)
   The .edu TLD is limited to specific higher educational institutions such as, but not limited to, trade schools and universities. Some institutions do not meet the current registration criteria have grandfathered domain names.
6. .gov (U.S. national and state government agencies) administered by General Services Administration (via Verisign)
   The .gov TLD is limited to United States governmental entities and agencies and qualifying state, county and local municipal government agencies.
7. .mil (U.S. military) administered by United States Department of Defense The .mil TLD is limited to use by the United States military.