D-Bus is a message bus, used for sending messages between applications. Conceptually, it fits somewhere in between raw sockets and CORBA in terms of complexity.
D-Bus supports broadcast messages, asynchronous messages (thus decreasing latency), authentication, and more. It is designed to be low-overhead; messages are sent using a binary protocol, not using XML. D-Bus also supports a method call mapping for its messages, but it is not required; this makes using the system quite simple.
D-Bus is still under heavy development, but is expected to be widely used. It comes with several interfaces, including GLib. See the description of libdbus-glib-1-2 for more information about those.
The daemon can be found in the dbus package.