In a nut-shell, these suffixes are issued by the various registering authorities around the world. The most common are those of the USA. The regulatory body for US registrations is the InterNIC. The following are their most common suffixes.COM Designed for commercial entities. .com rapidly became the top-level domain for business. Today, anyone can register a Web Address in the .com domain.
EDU is for 4-year, degree granting colleges/universities.
.NET Designed for organizations directly involved in Internet operations, such as network providers and network operation centers. Today, anyone can register in the .net domain.
.ORG Designed for miscellaneous organizations, including non-profit groups. Today, anyone can register in the .org domain
In the above example which has a .co.uk domain extension, the .uk part specifies that the country designation in the UK (United Kingdom) and the .co part specifies the type commercial, intended for commercial businesses. There are thousands of domain extensions in use around the world. Although there are restrictions on the use of the majority of domain extensions (for example a country's regulations may stipulate that you must own a business with permanent residency in that country in order to use a domain extension belonging to that country), there are are many domain extensions for which there are little or no restrictions.