What is D-BUS?
D-BUS is an IPC mechanism for sending and receiving messages across a common communications channel. At the protocol level D-BUS can be used as a peer-to-peer message transport for applications to communicate directly with each other. The real power of D-BUS comes from the bus daemons which act as routers for messages. There are two standard buses that a developer can rely on always being around. These are the system bus and the session bus.
The system bus is a global daemon that any application running in any context can use as a transport. It is a single point where applications can export services that anyone can use. Only one system bus daemon can be run at a time.
The session bus is a bus local to the current user’s session. It is used for communication between applications running within the same X session. For every login to X, a session bus daemon is started.