According to the Apple Developer Connection, Bonjour enables computers, devices and services on IP networks to automatically discover each other. This "automatic discovery" is built into all Mac OS X computers, and allows Macs to automatically discover Windows PCs that also have Bonjour installed on them.
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Bonjour is also known as "zero-configuration networking," meaning that as long as two computers are on the same network or networks, they can connect with each other without the need to go through the standard Internet routes.
In order for Bonjour to work, the two devices or computers must be able to allocate IP addresses without a DHCP server, translate between names and addresses without a DNS server and locate or advertise services without a DNS server.
The popular iTunes music application uses Bonjour, which allows users to share their music libraries with each other.
The Bonjour instant messaging application can be used to allow two computers to send instant messages directly to each other (without the need for the Internet), as long as the computers are on the same network.
While Bonjour doesn't come preinstalled on Windows, it can be downloaded from the Apple Developer Connection site at http://support.apple.com/downloads/Bonjour_for_Windows. One caveat is that Bonjour for Windows is 64-bit only.