Welcome to dhcpcd-dbus
dhcpcd is DHCP client. DBUS is an IPC mechanism. Add them together and you get dhcpcd-dbus!
dhcpcd-dbus receives interface configuration events from the dhcpcd control socket and emits them to the DBus listeners. dhcpcd-dbus also has methods to release, rebind, stop, query and configure dhcpcd on an interface. This allows users to control dhcpcd to some extent as all dhcpcd opertaions require root privilege and DBus has a fine grained ACL list for accessing these functions which dhcpcd-dbus can optionally use.
dhcpcd-dbus also listens to wpa_supplicant for wireless interfaces via it's control socket. This means that wpa_supplicant does not have to be compiled with DBus support, which is the case a lot of distributions.
dhcpcd-dbus is pretty complete right now. However, wpa_supplicant support needs to be improved so that we can connect to an AP with the correct credentials. Patches are always welcome :)
Of course, to the end user, dhcpcd-dbus by itself is useless. Luckily, dhcpcd-ui has a nice GTK+ front-end for monitoring dhcpcd.
