A domain is used in a company that has a website and email addresses for employees that are linked to the web site. The domain is owned by the individual business and is kept up by them. Search engines include domain names in their lists to be able to offer users the ability to link to and find any domain.
The domain is administered as one unit with rules that are set within the domain and all operations follow certain patterns within it as determined by the business. An example of this is when a person orders something from a domain which then sends an email within the domain to a member of the company to facilitate the order. The order being placed sparks the passing of the email through the domain. Domain names must be distinct from one business to the next and are often created using the name of the business or the industry they work within.