D-Bus is an IPC (inter process communication) technology designed to allow applications to interoperate without requiring tight coupling.
For more information about D-Bus itself see its website: http://www.freedesktop.org/wiki/Software_2fdbus
The Qt3 D-Bus bindings described here are a Qt3 style API around the base implementation to enable Qt3 developers to use D-Bus in their applications without requiring them to know about the details of the C based D-Bus API.
The two main use cases are:
Of course an application can do both at the same time.