Ultrapeers are users that you connect to in order to search.

To become an ultrapeer, users need to have a reasonably fast connection & not be firewalled.

Here's an explanation (click on link): GnuFU en - Gnufu

And another one: Ultrapeers - LimeWire

Advanced Tools shows you which hosts you're connected to: if your status is a leaf then it shows which UPs you're connecting to, if your status is ultrapeer it will show you which other UPs (shows as peers) & leaves (people who might have a slower connection, shorter uptime or a degree of firewall) that you're connected to.

If your status is a leaf then it can be useful to see which UPs you're connecting to... some UPs are 'bad' & connecting to those will result in seeing more spam results. So, being able to disconnect from those hosts can result in seeing better results. See this post (click on link)for more info: http://www.gnutellaforums.com/351053-post3.html