A top-level domain (TLD) is the last part of an Internet domain name; that is, the letters that follow the final dot of any domain name. For example, in www.company.co.uk the part 'uk' is the Top Level Domain.

Management of most top-level domains is delegated to responsible parties by the Internet Corporation for Assigned Names and Numbers (ICANN), which operates the Internet Assigned Numbers Authority (IANA) and is in charge of maintaining the DNS root zone. New TLDs for Geographical Names are under consideration.

Differerent groups of TLDs

ccTLD

Country code top-level domains (ccTLD): Used by a country or a dependent territory. It is two letters long according to the ISO 3166-1 norm.  Worldwide there are 252 ccTLDs.

Examples:

• .be (Belgium)
• .nl (Netherlands)
• .sr (Surinam)
• .eu (Europe)

ccSLD

A country code second-level domain (ccSLD) is a second-level domain to a country code top-level domain. It may be set apart by the domain name registry for the registration of third-level domains or registered to a third party as a subdomain. Many registries reuse common top-level domain names at the second-level such as "com", "net" and "org", e.g. .com.au.

Examples:

• .co.uk (commercial organisations in the UK)
• .com.tr (commercial organisations in Turkey)
• .org.il (Non-profit organisations in Israel)
• .edu.au (Educational institutions in Australia)
• .gov.ru (government organisations in Russia)

gTLD

A generic top-level domain (gTLD) is a top-level domain not belonging to a certain country but open for registration for all individuals or organisations. These are three or more letters long, and are named for the type of organization that they represent (for example, .com for commercial organizations). There are two groups of generic TLDs, sponsored sTLDs and unsponsored uTLDs.

 Examples of sTLD's:

•  .aero - aerospace and transport 
•  .asia - Asia countries, including Australia
•  .cat - Catalan language and culture
•  .coop - cooperative organisations
•  .edu - American educational institutions
•  .gov -  American government
•  .int -  international organisations, very strict policy by IANA
•  .jobs - recruitment and human resource companies
•  .mil -  American military
•  .mobi - mobile communication
•  .museum - musea 
•  .tel - internet telecommunication
•  .travel - travel agencies

Examples of uTLD's:

• .biz - business
• .com - commercial 
• .info - informative
• .name - individual persons
• .net - network
• .org - non-profit organisation 
• .pro - professions

iTLD

An infrastructure Top Level Domain (iTLD) is used exclusively for Internet-infrastructure purposes and internet protocols like the reverse mapping of IP-addresses and ENUM phone numbers.

Example:

• .arpa  - the only one active (Address and Routing Parameter Area)