The motherboard, CPU(s), memory, power supply units.
Motherboard (a form of PCB) to provide mechanical support for other system devices;
Processor or Coprocessor (CPU) to execute program instructions;
Random Access Memory (RAM) to store the program instructions and data residing in the computer’s main memory while it is being used;
A connector that allows data signals between components on a circuit board and their corresponding pins on connectors accessible at the back panel of the case. Every component needs power, so there are several different types of power supplies that range from small, single-output boards provided only with +3.3 V to larger multisource units providing numerous voltages plus higher currents.