Officially dubbed "Ultrapeers", it reliefs its connected nodes from the Gnutella traffic, by answering Gnutella messages on their behalf.

Normally spoken, Gnutella nodes are contsantly jabbering. You'll notice the constant load on your modem when using Gnutella. This can reduce your total available bandwidth, which is obviously not a good thing.

But with Ultrapeers, this is different. When you connect to an Ultrapeer, you become a "leaf node". You tell the ultrapeer "I have these files, forward search messages to me that (approximately) match these files only". From that point, you are shielded from irrelevant search messages, pings, pongs, traffic from other nodes, etc. (note: once you're a leaf node, you usually don't accept other nodes connecting directly to you - they are told by your client about the Ultrapeer, which they might try instead). The ultrapeer will handle all messages for you. You can still send a search yourself, and search messages for a file you might indeed have are forwarded to you (and processed as usual).

This does mean that Ultrapeers have the additional burden of managing all of its leaf nodes' messages. For that reason, Ultrapeers are selected only when certain criteria. Such as: Does the operating system handle many connections? Is it a fast computer? Does it have a fast Internet connection?

Leaf nodes are best meant for modem users, but current Gnutella clients implementing Ultrapeers allow just about anyone to become a leaf node.

Now, this is in laymen's terms and probably not 100% accurate. If you want to read the more technical details about it, visit the followin URL:

http://www.limewire.com/developer/Ultrapeers.html