The top-level domain (TLD) of a URL is indicated by the characters which form the last part of a domain name. For example, in the case of CountryCodes.info the TLD is ".info". The 2 most common types of TLDs are 'generic' and 'country'.
Generic top-level domains (gTLD) include the likes of ".com", ".net", ".info" etc. These are not specific to any particular country or region. More Info
A country code top-level domain (ccTLD) is a name with a 2 letter code appended to the end which denotes a domain reserved for a country, soverign state or a dependant territory. ccTLDs are based on ISO 3166-1 two-letter country codes. For example, www.google.se is the Swedish URL for Google, whereas www.google.com.au is the Australian site ('se' being the country code for Sweden and 'au' for Australia).
Some ccTLDs have been marketed as having meanings other than just their country of origin in order to increase the uptake of the domain and create income for the country/organisation controlling it. Examples include '.ws' which is the ccTLD for Western Samoa but is marketed as standing for "web site", and '.co' which belongs to Colombia but is marketed as an alternative to the generic '.com' TLD.
The Internet Assigned Numbers Authority (IANA) is responsible for the operations and maintenance of many key aspects of the Domain Name System (DNS) including determining the appropriate trustee for each ccTLD.More information on ccTLDs can be found at Wikipedia.