dbus (short for Desktop Bus) is an inter-process communication (IPC) mechanism: a message bus that allows different processes and applications to communicate with each other. All processes connected to the bus can communicate with each other through it. <code>dbus</code> is a spec, <code>libdbus</code> is a reference implementation of the specification.
<ul>
 <li><code>dbus-x11</code> is an older implementation of dbus that creates a new <code>dbus-daemon</code> for each graphical display that is opened (i.e. when a user logs in). This <code>dbus-daemon</code> (and any of its services) dies whenever that graphical session ends.</li>
 <li><code>dbus-user-session</code> instead has a <code>systemd</code> user service, where the <code>dbus-daemon</code> is shared among all of a user's sessions (graphical, ssh, tty, etc). This <code>dbus-daemon</code> dies when the last of a user’s login sessions has ended. <code>dbus-user-session</code> has a hard dependency on <code>systemd</code> which only supports running one graphical session per user at a time.</li>
</ul>
Usage:
<ul>
 <li><code>systemd</code>: uses the D-Bus wire protocol for communication between <code>systemctl</code> and <code>systemd</code>.</li>
 <li><code>NetworkManager</code> / <code>udev</code>:
 <ul>
 <li><code>udev</code> daemon creates an entry for each network interface card (NIC) installed in the system in the network rules file.</li>
 <li>D-Bus signals the presence of a new network device—wired or wireless—to <code>NetworkManager</code>.</li>
 <li><code>NetworkManager</code> then listens to the traffic on the D-Bus and responds by creating a configuration for this new device.</li>
 </ul>
 </li>
</ul>

What is dbus?