From AMule Project FAQ
Servers on the eD2k network
On the eD2k network, when connecting, you connect to a server (read this to know more about it).
This server provides information such as the search results, the clients who are sharing specified files, the files transfer (when on LowID). and such.
On this network, the server is a central machine which you connect to (you can choose which server to connect to). Most servers are interconnected to each other so that global searches can be done.
Servers on the Kademlia network
On the Kademlia network there are no servers. Or, lets say, each client is, at the same time, a server.
The description on how this network works can be found here.