DNS (Domain Name System) makes the internet easier to use. You have not to use an IP address like 220.127.116.11, but you can use an easy-to-remember name, like "www.website.com". More info is available on Wikipedia
Dynamic DNS allows users to change the association of names to IPs dynamically.
Dynamic DNS and HFS
Most of the dynamic DNS services support updating the IP via a special URL. HFS uses this feature to keep your dynamic DNS referring to the correct, current IP address.
Dynamic DNS can also be used to hide your IP address from others. They only see the name, for example myserver.dyndns.org, and not the IP you are on.