Domain names are composed of at least two levels, a top-level domain and a second-level domain. The top-level domain, also referred to as TLD, is the extension attached to Internet domain names. There are a limited number of predefined TLDs, such as .com, .net, .biz, .org and .co.
The second-level domain name includes the top-level domain and the domain name itself. For example, "microsoft.com" is a second-level domain, as it includes the domain name "microsoft" and the top-level domain ".com."
The third-level domain name is used to refer to different servers within different departments of a company. In larger organizations each division or department might have its own third level domain, which can serve as an effective means for identifying it. www is the default third-level domain name and the most commonly used. The third level domains are not mandatory unless the user has a specific requirement.
For example: www.furniture.net is a third-level domain. .net is a top-level domain and furniture is the domain name. The www is third-level domain.
Third level domains should not be confused with the part of e-mail addresses that appears to the left of the at (@) symbol. Thus, for example, the support in support@furniture.net is not a third level domain, but rather is merely a specific addressee or mailbox at an Internet address.
