D-Bus is a message bus system that provides an easy way for inter-process communication. It consists of a daemon, which can be run both system-wide and for each user session, and a set of libraries to allow applications to use D-Bus.
D-Bus is enabled automatically when using systemd because is a dependency of systemd.
Starting the user session
1.10.0-3, the D-Bus session is started automatically. 
its homepage for more info.is an easy to use D-Bus debugger GUI tool. D-Feet can be used to inspect D-Bus interfaces of running programs and invoke methods on those interfaces. See