(co.uk or .com etc). Domain extensions, or suffixes, are categories of Internet domain names, and serve to describe the type of company, organisation, country or other category. The technical term used for domain extensions is top-level-domain (or TLD). There are many categories from which to choose, and some extensions are more popular than others. Be aware that for the majority of domain extensions, some exceptions apply, the category the domain claims to represent is not enforced.