D-Bus is a message bus system, which allows client programs to call procedures on a service - basically, the machine-local equivalent to XML-RPC and SOAP.
D-Bus is a message bus system, designed to provide ways for programs to communicate with each other. D-Bus uses remote procedures to allow a client to call methods on a "service", and is conceptually similar to XML-RPC and SOAP. D-Bus, however, is designed for use in applications running on a single system. The FAQ offers a comparison to other well known IPC systems, as well as other general questions.
D-Bus is, in addition to an implementation, a specification - the document itself is freely available. In addition, an Introduction To D-Bus gives a high-level overview of how D-Bus is designed.
D-Bus, although used on POSIX systems (usually Linux and the *BSDs), has been ported to Windows and has been officially added to the specification.
D-Bus related questions will typically be how do I accomplish [X] in D-Bus - D-Bus is technical enough that most any question relating to it will be on topic.
