The Dark Brotherhood worships and reveres the Dread Father Sithis and the Void, with the Unholy Matron, the Night Mother, being his bride.
When someone performs the Black Sacrament, the Night Mother is informed of their intentions, and the location of the client is relayed to the Listener, who then relays that information to the four Speakers. The Speakers then assign assassins to the contracts to be completed.
Together, the Listener and the Speakers form the Black Hand, the ruling body of the Dark Brotherhood: four fingers and a thumb.
Silencers are assassins who are not bound to any sanctuary and answer directly to the Speakers. They perform particularly dangerous or sensitive contracts and missions, and are akin to the nails or talons of the Black Hand.
The Keeper is a special position instituted by the Black Hand in the event of the Night Mother’s death. The Keeper is charged with taking care of the Night Mother’s corpse to preserve her body and retain her spirit on Tamriel, so she can continue to accept contracts and relay information to the Listener.