Question: What is Dynamic DNS (DDNS)?
Dynamic DNS allows you to have fixed domain name points to dynamic public IP, means the domain name will point to new IP when the IP is changed. This service is particular useful for hosting web server, mail server or public server in home network that use Cable/DSL broadband connection with dynamic public Internet IP, then the Internet users can just connect to your dynamic DNS and don’t worry about the public Internet IP change.
Usually you will need to have small DDNS update client installed on your computer in order to notify the DDNS server when there is public IP change. If you router supports this DDNS client feature, then you can just enable this feature on router and don't need that DDNS update client.
The examples of Dynamic DNS service providers are www.dyndns.org and Dynu DNS.