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 suffix or extension attached to Internet domain names. There are a limited number of predefined TLDs, such as .com, .net and .org.
A second-level domain (SLD) 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, "microsoft.com" is a second-level domain, as it includes the domain name "microsoft" and the top-level domain "com."
If you need to further distinguish your second-level domain name, you can use a third-level domain name, such as "owa.msoutlookonline.net." Typically a third-level domain name is used to refer to different servers within different departments of a company. In this case, owa.msoutlookonline.net points to Outlook Web Access for Exchange customers of msoutlookonline.net.