Bonjour is an Apple created open standard for Zero configuration networking (zeroconf). The idea behind it is that devices on a network are able to find each other without manually setting up an IP, or entering information ahead of time.

For iTunes, it uses Bonjour to:

1. Share music/movies/etc to other iTunes users on the same network.
2. Pipe music to any Airport Expresses on the network.
3. Discover Apple TV units.
4. Allow an iPhone or iPod touch to remote control iTunes.

Outside of iTunes, Bonjour on Windows will:

1. Allow a network printer to be used without configuration.
2. Add network bookmarks to Safari or IE. Most printers advertise their web config over Bonjour, among other things.
3. Allow discovery and configuration of any Apple router.

Other products may be using Bonjour for related tasks. Adobe software for example will use it to find an Adobe Version Cue server on the network.

If you have no use for any of the above, feel free to disable the service. While it shouldn't be causing any delays, Tony seems to indicate he has seen them on Vista due to Bonjour.