We need:
a fuel such as U-235
a moderator to thermalize (i.e., slow down) the fast neutrons
a coolant to remove the heat
a control system to control the number of neutrons
a shielding system to protect equipment and people from radiation
a system that pulls all this together into a workable device.
Lets look at these requirements in turn to gain some insight on how and why CANDUs are built the way they are.