A domain name “extension” is called a “top-level domain (TLD).” These are divided into three different categories: generic top-level domains (gTLD), country code top-level domains (ccTLD) and infrastructure top-level domains.
Generic top-level domains are the ones most frequently seen on the Web:
.com (originally intended for use by commercial organizations but is available to anyone).
.net (originally intended for use by sites directly related to the Internet but is available to anyone).
.org (originally intended for use by non-profit organizations but is available to anyone).
.edu (used by educational organizations).
.gov (reserved for agencies of the United States government).
.mil (reserved for the United States military).
.int (reserved for international organizations established by treaty. i.e. the European Union: http://europa.eu.int).
.aero (reserved for members of the air transport industry).
.biz (for use by businesses only).
.coop (reserved for cooperative associations).
.museum (reserved for museums).
.name (reserved for individuals).
.pro (being developed for professionals and related entities).