Libavahi-common: contains some functions used by the Avahi-daemon and the mDNS stack.
Libavahi-core: implements a flexible mDNS/DNS-SD stack and is highly used by developers.
Avahi-daemon: implements the MDNS stack by using Libavahi-core.
Libavahi-client: facilitates the use of DBUS API by hiding DBUS internals.