Domain names or Top Level domains (TLD) as they ar more formally refered to is the last part of an Internet doamin name and are used to resolve Internet Protocol (IP) adresses to easier to remeber alphabetical equivilants.
The Internet Assigned Numbers Authority (IANA)classifies top-level domains into 2 types for mainstream use: Country code top-level domains (ccTLD)are used by countries and dependent territories. These are 2 letters, for example .es for Spain or .gi for Gibraltar. Generic top-level domain (gTLD) are used (theoretically) by a particular class of organization, example, com for commercial organizations. these are 3 or 4 letters.
Most gTLDs are available for use worldwide, but for historical reasons .mil for military use and gov for governmental use are restricted by the respective U.S. authorities. gTLDs are subclassified into sponsored top-level domains (sTLD), e.g. .aero, .coop and .museum, and unsponsored top-level domains (uTLD), for example .biz, .info, .name and .pro.