lol Ultrapeers are computers that help you to connect to the gnutella network. They also filter out unnecessary traffic. So only your searches, uploads, downlds are what you're interested in when in leaf mode. The gnutella decides when & who becomes an ultrapeer which may depend on users free bandwidth & speed & whether more UP's are needed. Ultrapeers help people connect to the network which is more valuable than uploading files. So don't feel bad about being a leaf. This will give a bit of a description of the terms http://www.limewire.com/english/content/glossary.shtml

If you want a better description (sorry mine's a bit dry) then ask.