The TLD is the part of the domain name after the dot and which identifies the purpose of the domain name.
The most popular type of TLD are .com and .net.
Generally the TLD identifies what type of organization uses the domain but these are not rigidly adhered to. For example .com is generally used by companies and commercial enterprises, .net is used by internet or online businesses and .org tends to be for organizations, charities, not for profit and Non Governmental Organizations (NGO).
Generally there are no restrictions on who can register TLDs.
What are restricted top-level domains (rTLDs)?
rTLDs are TLDs that are used for specific purposes and have restrictions on who or which organizations can register them. This is to ensure that they can only be registered by members of a certain community or type of institution. Common examples are .museum and .edu.
What are Country Code TLDs (ccTLD)?
ccTLDs are reserved for specific countries and are the two letter suffix at the end of a domain name. For example, for the UK the ccTLD is .uk and for Germany it is .de.
What are Country Code Second-Level Domains (ccSLD)?
If you are using a ccTLD (e.g. .uk) and you are a business then you might register a .co.uk domain name as they are popular, easy to memorize and low cost. In this case the .uk is the ccTLD and the .co is the ccSLD.
Who Can Register a ccTLD?
It all depends on the type of domain name and the country. Some ccTLDs can be registered by anyone such as .tv (Tuvalu) or .co (Columbia). They are issued on a first come first served basis.
However some countries or regions will have residency restrictions such as .eu domains.