The IP address for a domain is stored in what is called the DNS A record , but the DNS can store other information about a domain.
For example, a domain may have several CNAME records. (That is a geeky abbreviation for "canonical name"). These assign aliases. For example, you may have a Web page that is hosted at a service like Google Apps, and wish to refer to it as www.mydomain.com. You could do that by entering a CNAME record in the DNS for your domain that equated the two:
CNAME www => ghs.google.com.A request for a URL containing www.mydomain.com would be redirected to Google.
To fully customize a Google Apps identity, you could add additional CNAMEs:
CNAME mail => ghs.google.comThe DNS also uses MX records to redirect requests by email clients.
CNAME calendar => ghs.google.com
CNAME start => ghs.google.com
CNAME docs => ghs.google.com
Adding or modifying MX, CNAME and other DNS entries requires access to the DNS for your domain. Your domain registrar might allow you to change the DNS yourself or might require that you request that they make changes for you.
You can see an example of a DNS configured to direct requests for various services at larrypress.org here.
You can query the DNS to see current entries for a domain using the nslookup client that is built into Windows. That client has a command-line interface. There are also many Web sites with graphical user interfaces for DNS queries. One is a service of Kloth.net. The offer GUIs for queries using whois, nslookup and dig a popular upgrade to nslookup.