Domain name you are familiar with 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 suffix or extension attached to Internet domain names. There are a limited number of predefined TLDs, such as .com, .net, .org, .biz, .us, etc.

A second-level domain is the portion of a Uniform Resource Locator (URL) that identifies the specific and unique administrative owner associated with an Internet Protocol address (IP address). The second-level domain name includes the top-level domain name. For example, "mivamerchant.com" is a second-level domain, as it includes the second-level domain name "mivamerchant" and the top-level domain "com."   Most of your websites will use second-level domains.  When you register a domain name, for all but a few special cases, you are registering a second level domain name.

A third-level domain is commonly referred to as a "subdomain", and adds a third domain portion to the URL.  The most common third level domain is "www", such as, "www.domain.com", or "www.mivamerchant.com".    You will also see specialized third-level domains in use on some sites, such as "secure.domain.com", "blog.domain.com", etc.    Once you have registered a regular second-level domain name, you may use any third-level domain name that you want.  For example, if you registered "mydomain.com", you may also use "www.mydomain.com", "secure.mydomain.com", "anything.mydomain.com", and so on.