An ultrapeer is what leafs connect to, and direct most of the traffic as efficiently as possible.

UP's basically communicate with each other and look after a set of leafs (max 30 in LW).

The UP handle most of the message traffic, and shield the leafs from irrelevant queries, only sending requests to a leaf if they have the file and are likely to be able to deliver it (they're not too busy)

This frees up the leafs from getting flooded with requests that could otherwise clog the system.

You can see your details in the options-> bug reports->view example if you scroll down a bit

Uptime: 2:31
Is Connected: true
Number of Ultrapeer -> Ultrapeer Connections: 24
Number of Ultrapeer -> Leaf Connections: 0
Number of Leaf -> Ultrapeer Connections: 0

If you need more info, I can probably dig up some links. I haven't checked the LW FAQ for a while, but it should give you the basics.

btw-with 2.5 minutes of uptime, a what's new image search with 24 peers and no leafs yet got me 498 results.

So no leafs were needed to get results when running as an ultrapeer.