Domain Name - A name that identifies one or more IP addresses, usually on a web and/or email server. For example, the domain name microsoft.com represents one or many IP addresses (70.90.126.121).

If you own the domain prefix (microsoft.com), you also own all suffix (microsoft.com/contact) variations for your website and email addresses (joe@microsoft.com).

Every domain name has a suffix that indicates which top level domain (TLD) it belongs to. There are only a limited number of such domains. For example:

.com - Commercial Business
.gov - Government Agencies (congress.gov)
.edu - Educational / University  institutions (msu.edu)
.org - Organizations, Typically non Profit (mothertherese.org)
.net - Network Organizations