What is it?
Bonjour provides a general method to discover services on a local area network. The software is widely used throughout Mac OS, and allows users to set up a network without any configuration. It is used to find printers and file-sharing servers. iTunes uses Bonjour to find shared music, iPhoto to find shared photos. Bonjour for Windows and AirPort Base Stations may be configured to use Wide Area Bonjour which allows for wide area service discovery via an appropriately configured DNS server.
Description of jdns_sd.dll from Apple
“Bonjour, also known as zero-configuration networking, enables automatic discovery of computers, devices, and services on IP networks using industry standard IP protocols. It is a key component of Apple applications (e.g., iTunes, iPhoto), services (e.g., MobileMe) and devices (e.g., Apple TV, and Airport). Developers can easily leverage Bonjour from both Mac OS X and iOS.”