DNS is a hierarchical naming system. A DNS name includes the names of all of the DNS namespaces that it belongs to. The DNS namespace is made up of a number of components.
The DNS namespace starts with a root domain. The root domain is a logical domain that has no name and is represented by a dot (.) The root domain holds all top-level domains (TLDs) because they are the highest-level end of the DNS namespace. Examples of TLDs include .com, .net, .ca etc....
In order to have an Internet presence you have to register an Internet domain name. This domain name will be a subdomain of a top-level domain. Examples of a registerd domain name include Thelazyadmin.com, Microsoft.com and Google.com.
The components mentioned make up the External DNS namespace. On the inside of an organization you may have an Internal DNS namespace. This namespace contains the subdomains of the registered domain name as well as servers within the registered domain. An example of a subdomain would be blogs.msdn.com, or an example of a server would be mail.dortoh.ca.
Internal DNS namespaces can be, but do not have to be, subdomains of a public Internet domain name. You can create an internal namespace using a non-public TLD of your own creation. Some examples of this would include .local, .lab or .myowntld.