A domain name locates an organisation or other entity on the Internet. For example, the domain name www.totalbaseball.com locates an Internet address for "totalbaseball.com" at Internet point 199.0.0.2 and a particular host server named "www".
The "com" part of the domain name reflects the purpose of the organisation or entity (in this example, "commercial") and is called the top-level domain name.
The "totalbaseball" part of the domain name defines the organisation or entity and together with the top-level is called the second-level domain name. The second-level domain name maps to and can be thought of as the "readable" version of the Internet address.
A third level can be defined to identify a particular host server at the Internet address. In our example, "www" is the name of the server that handles Internet requests. A second server might be called "www2". A third level of domain name is not required. For example, the fully-qualified domain name could have been "totalbaseball.com" and the server assumed.

Subdomain levels can be used. For example, you could have "www.nyyankees.totalbaseball.com". Together, "www.totalbaseball.com" constitutes a fully-qualified domain name.

Host names are organised based on the tree structure of DNS. The individual node name comes first, followed by the domain where it resides, followed by the domain in which the domain resides, and so on, with each level separated by a dot.
So, for example, when we see the host name host1.engineering.cisco.com, we know that the node host1 is in the engineering subdomain of the cisco domain, which in turn is in the com domain, which is under the root domain of the Internet.

On the Web, the domain name is that part of the Uniform Resource Locator (URL) that tells a domain name server using the domain name system (DNS) whether and where to forward a request for a Web page. The domain name is mapped to an IP address (which represents a physical point on the Internet).
More than one domain name can be mapped to the same Internet address. This allows multiple individuals, businesses, and organisations to have separate Internet identities while sharing the same Internet server.
To see the IP address for a domain name, use the ping command

ping url.

It may be worth noting that the domain name system contains an even higher level of domain than the top-level domain. The highest level is the root domain, which would be represented by a single dot (just as in many hierarchical file systems, a root directory is represented by a "/" ) if it were ever used. If the dot for the root domain were shown in the URL, it would be to the right of the top-level domain name. However, the dot is assumed to be there, but never shown.