D-Bus is an inter-process communication mechanism for applications residing on the same host. The communication is based on messages. Data in the messages is carried in a structured way, it is not just a byte stream.
The communication is connection oriented to two kinds of message buses: a so called system bus, and a session bus. On a given machine, there is always one single system bus for miscellaneous system-wide communication, like changing of hardware configuration. On the other hand, the session bus is always related to a single user's session.
Every client application, which is connected to a bus, registers under a unique name at the bus. This name is used for identifying the client application. Such a unique name starts always with a colon, and looks like ‘:1.42’.
Additionally, a client application can register itself to a so called known name, which is a series of identifiers separated by dots, as in ‘org.gnu.Emacs’. If several applications register to the same known name, these registrations are queued, and only the first application which has registered for the known name is reachable via this name. If this application disconnects from the bus, the next queued unique name becomes the owner of this known name.
An application can install one or several objects under its name. Such objects are identified by an object path, which looks similar to paths in a filesystem. An example of such an object path could be ‘/org/gnu/Emacs/’.
Applications might send a request to an object, that means sending a message with some data as input parameters, and receiving a message from that object with the result of this message, the output parameters. Such a request is called method in D-Bus.
The other form of communication are signals. The underlying message is emitted from an object and will be received by all other applications which have registered for such a signal.
All methods and signals an object supports are called interface of the object. Interfaces are specified under a hierarchical name in D-Bus; an object can support several interfaces. Such an interface name could be ‘org.gnu.Emacs.TextEditor’ or ‘org.gnu.Emacs.FileManager’.