Hello,
It maps a dynamic IP address of say, a cable modem user, to a domain name on the Internet. This makes it possible for the person to give the domain name out to friends instead of an IP address. If you do not have a static IP address and are running a service from your home network like a TeamSpeak server or a gaming server, having a dynamic IP address sucks. It means every time it changes you have to tell all of your users the new IP.
Services like
www.no-ip.com, www.dns2go.com, and
www.dyndns.org give you a FREE service that lets you map a dynamic IP address of yours to a domain name of theirs like dsm.servegame.com, and then all you have to give your friends is dsm.servegame.com, not an IP address. Then when your IP changes, a client program installed on your server computer updates the IP on the NO-IP side of things. The user never needs to change anything again as long as the NO-IP type service is up and working.
If any of you used roger wilco in the past, you might remember how much of a pain in the arse it was to always figure out what IP address your friends were on. Well, such services as explained above, solve the problem. When your friends and family have a hard time connecting to your servers, they simply will not use them!
I hope this explains it. Frankly, I'm surprised not more people on here have their own gaming servers or TeamSpeak servers!
;-)