This is much easier to understand, read full text at:
http://advancedhomeserver.com/dynamic-dns-ddns-and-a-little-more/
DDNS is a technique that keeps on top of the IP address your ISP assigned to you. Periodically, and fairly often, a program runs to find out what your ISP provided internet IP address is at this moment. Then a private company somewhere on the internet is contacted and associates your current IP address with an internet name you provided to them. The internet name is a composite of a name you chose and their actual domain name. When someone on the internet enters that name in a browser, they’re talking to you.
For example, Asus offers a free DDNS service for buyers of some of the products they sell. I own an Asus router and use their DDNS service. Within the router, I turned the DDNS feature on and entered a name. Asus agreed the name was unique and established the link within a couple of seconds. The router relays my current IP address to Asus DNS servers several times a day. I now have an accessible internet presence at ________.asuscomm.com.
