Software.

Bonjour is a general method to discover services on a local area network. It is widely used throughout Mac OS X and allows users to set up a network without any configuration. Currently it is used by Mac OS X and on other operating systems to find printers and file sharing servers. It is also used by iTunes to find shared music, iPhoto to find shared photos, chat, Adobe Systems Creative Suite 3, Proteus, Adium, Fire, Pidgin, Skype, and the Gizmo Project to find other users on the local network, TiVo Desktop to find digital video recorders and shared media libraries, SubEthaEdit and to find document collaborators, and Contactizer to find and share contacts, tasks and events information. Additionally it is used by Safari to find local web servers and configuration pages for local devices, and by Asterisk to advertise telephone services along with configuration parameters to VoIP phones and dialers.
Software such as Bonjour Browser or iStumbler can be used to view all services declared by these applications and more. Also, Apple's "Remote" application for iPhone and iPod Touch uses Bonjour to establish connection to iTunes libraries via Wi-Fi. Without special DNS configuration, Bonjour only works within a single broadcast domain, which is usually a small area.
Bonjour is sometimes misunderstood to make services on a personal computer (for instance, file sharing) available to the public Internet, which could be considered a security risk. In fact, Bonjour does not provide any extra access to services, even on the same local area network (LAN); it merely announces ("advertises") their existence. For example, a user can browse a list of nearby computers which share files—Bonjour on these computers has told the user that the service is available—but he or she must still provide a password to access any protected files on these machines. Additionally, Bonjour works only in a close range; by default, its messages only reach users of the same link.
Bonjour services are implemented at the application level largely using standard TCP/IP calls, rather than in the operating system. Although Mac OS X provides various Bonjour services, Bonjour works on other operating systems. Apple has made the source code of the Bonjour multicast DNS responder, the core component of service discovery, available as a Darwin open source project. The project provides source code to build the responder daemon for a wide range of platforms, including Mac OS 9, Mac OS X, Linux, *BSD, Solaris, VxWorks, and Windows. In addition, Apple provides a user-installable set of services called Bonjour for Windows as well as Java libraries. A number of Windows programs use Zeroconf, including Adobe Systems Creative Suite 3, iTunes, Cerulean Studios' Trillian Pro 3 Ruckus Music Player from Ruckus Network and the text editor.