D-BUS supplies both a system daemon (for events such as "new hardware device
added" or "printer queue changed") and a per-user-login-session daemon (for
general IPC needs among user applications). Also, the message bus is built on
top of a general one-to-one message passing framework, which can be used by
any two apps to communicate directly (without going through the message bus
daemon).

WWW: http://www.freedesktop.org/Software/dbus