"The Dynamic DNS feature allows you to host a server (Web, FTP, Game Server, etc...) using a domain name that you have purchased (www.whateveryournameis.com) with your dynamically assigned IP address"
Exactly like it says - the dynamic DNS feature allows you to host a server ... using a domain name ... with your dynamically assigned ip address - it does not say
on the DNS-323 and the only one of those servers mentioned that you can host on an unhacked DNS-323 is an ftp server.
Briefly - hosting a server would normally require a domain name to be registered and a static public ip address to point that domain name to - plus of course the hardware, software, etc, to host the server. Dynamic DNS allows you to register the domain name and have it point to a dynamically assigned public ip address.
Without this feature, hosting a server without a static public ip address becomes challenging, you need to keep track of when the public ip address changes and what it changes to, and pass the information onto all of your potential visitors.
The DNS-323 has a dynamic DNS client that can update the dynamic DNS server so that the domain name points to the correct ip address, and assuming that you have correctly configured your router and other hardware to host the server, you can host all of the above mentioned servers,
but not on the DNS-323.
The only one of the above mentioned servers that runs on an "out of the box" DNS-323 is the ftp server.