Here's a short part about the UP-Leaf model (copied from
http://gnufu.net ):
===Network model: Change who calls whom: Ultrapeers and Leafs===
====UPs really short====
You'll surely have friends who know very many other people, and whom you can ask, and be sure they'll know exactly the person who can give you the answer. These are called Ultrapeers in Gnutella. An Ultrapeer doesn't have to know much himself/herself, he/she just needs to know who knows it. In Gnutella that means that a good Ultrapeer doesn't need to have many files to benefit the network. If you're afraid to share much, you should become an Ultrapeer in Gnutella.
====Ultrapeers and Leafs more detailed====
In the Computer World, as in the Real World, there are contacts who can cope with more calls, and those who can't phone often (or can't afford the bills). In the Real World this is because they have more free time. Whereas, in the Computer World, it is because they have faster connections (Like DSL, Cable, T1, T3 or similar broadband).
Upon realizing this, the developers decided to change the topologiy, that means how the network looks from the outside when you draw it. Now you don't just call any of your friends, but only those whom you know have the time to take your call and to send it on to others. To save you from too many calls, they then ask you which kinds of informations you have or, to express it in a more human way, what your speciality is. In the Computer World that means your computer sends a list of all its files to the Ultrapeer, which is how we call these kinds of contacts. That list contains summaries (Hash-Strings) of all your shared files (those you decide to let others download) by which the downloader can verify that they are, indeed, the files he or she wants. Whenever a call reaches the Ultrapeer, it checks if you could know an answer and calls you only in that case.
These Ultrapeers have many connections to others, which means they have a really big address book. Normally they stay in contact with 16 other Ultrapeers whom they have in their address book and to whom they send questions, and who send them to 16 more, each. Also they have about 16 leafs, who can't or don't want to phone that much, from which they accept calls, and whose files or, for the human world, specialities they know.
This may seem like a foul bargain for the Ultrapeers, who devote far more resources to keeping the network intact than leafs, but in fact it isn't. While the Ultrapeer (UP) uses much of her time for keeping the network running, the leafs specialize on gathering and delivering information. So, when anyone, Ultrapeer or Leaf, wants to know something, he or she simply starts a call and a leaf specialist can explain it to them. That way people specialize to get more for all.