In a client-server network, there is one distinguished host which acts as the server, and all the clients connect to the server (not directly to each other). Therefore, if you were to draw a diagram of the network it would look like a star, with the server in the middle and connections radiating out to each client. In a peer-to-peer network, all hosts are of equal status, and may make connections arbitrarily to one another, so the graph of the network topology would have connections going every which way. The "master server" system is really just for when you have so much data that one machine can't act as the server - then you replace the "server" by its own client-server network with one "master server" and several "slave servers", all of which appear to the outside world (the original clients) as effectively a single entity.
