A crown is the peice that holds the stanchions together. A crown is the same thing as a triple clamp.
A single crown fork only has a single crown under the head tube. On a single crown, the steer tube is connected to the crown.
A dual crown or dual triple clamp fork has a lower crown under the headtube that can be moved up and down (in which the steer tube is connected to). The top crown then tightens to the top of the 3 tubes.