Expansion chassis usually have four components:
(1) Host card: PCI card that installs into the computer.
(2) Cable: cable which connects the host card to the expansion chassis.
(3) Backplane Controller card: the card that gets installed in the expansion chassis itself.
Note that the name "backplane" is not used by every expansion chassis manufacturer to refer to this card. Sometimes "backplane" denotes the motherboard in the expansion chassis.
(4) Chassis: the actual metal box that houses the circuit board and the PCI slots.
It is also important to note that some expansion chassis exist that do not have a separate card inside to connect to the expansion chassis cable. In these cases, the circuit board in the chassis has a connector built onto it for the cable, and the expansion chassis therefore only has three components (host card, cable and chassis).