To add on to what apollo said...
Bonjour is Apple's version of UPnP for the network. iTunes uses, as does iChat (which is nice if you're on a large LAN, you can see who else is on, without having to add people to a "buddy" list). Other application use it as well, like subethaedit, which gives users a collaborative document writing, coding platform, and you can see live edits and such.
There's even a windows bonjour client, so you can see printers hanging off a Mac, and so forth.
Poking at security since 1986. +++ATH