I do not pretend to understand exactly how the Gnutella network works. It is a nifty invention however
When anyone makes a search on the network this search reaches all connected Frostwires, Limewires, Phexes and any other Gnutella Clients. Some of this information gets uploaded to the network.
Put another way. If you make a search on Frostwire, you get file details show up in your search window. These results come from the people sharing the files, and not from a central server. Connected clients are continually sharing the information about their shared files, which does involve a certain small amount of uploading.
This is how I understand it... in non geek terms