First, a nitpick: The .com
part of a domain name is called the "top level domain" or TLD, not "extension".
There are a limited set of TLDs defined and recognized by IANA consisting of every two-letter country code and certain names with three or more letters. Originally, the generic names were just .gov
, .com
, .net
, .org
, .edu
, and .mil
(along with .arpa
for historical reasons, as well as defined unassigned names such as .test
and .example
).
Recently, the TLD name space was expanded with the names .biz
, .info
, and .name
among others, and a mechanism was created for sponsoring organization to apply for and create additional names. A complete list with notes can be found at Wikipedia, and IANA maintains the official list.
Which should you select? The one or ones that best fit your site's purpose and content. Pragmatically, if you pick anything other than COM
or NET
, you will probably want to consider also capturing the same name in COM
and/or NET
to prevent user confusion.
In the long run, the idea is that organizations would pick the best name and stick to it, and end users would be educated that not everything ends in COM
.
1Those "extensions" are called Top Level Domains or TLDs. – innaM – 2009-07-31T08:42:58.580
.xxx (is less than 15 characters long) – Andrew Grimm – 2009-08-03T09:14:45.487