Because there are a limited number of IPv4 addresses, when you connect to the Internet you are usually given a dynamic IP address. That is today you'll get address x and tomorrow you could get address y. This means that your computer, router, modem is connected to the Internet at a different point each time. That makes it difficult to find if your out on the Internet and want to "phone home" or run services that you want to make available to others - like a web or FTP server. You can get a static IP address but ISPs usually charge more for this.

Dynamic DNS fills the gap between the two.  Firstly you register a host name with them.  They have many domains available and you easy create something like overnet.homeunix.com - as I have done. When your Dynamic DNS enabled router or workstation connects to the Internet it contacts the main site and tells it the IP address it has been given by the ISP. The main site then updates its records so that when you access overnet.homeunix.com you are directed to your device.

Though the Dynamic DNS protocol is now used by a number of services Dynamic DNS themselves are the best service to use. OverNet usually configures routers to use Dynamic DNS to aid with diagnosing Internet connection problems and supplying remote support to it's customers.