The document discusses the major internal components of a computer system unit. It describes the processor, memory, expansion cards, ports, and buses that allow communication between components. The processor, called the central processing unit or CPU, contains an arithmetic logic unit for processing and a control unit for directing operations. Memory, which comes in volatile RAM and non-volatile ROM, temporarily stores data and instructions for processing. Expansion cards add capabilities through slots on the motherboard. Ports connect external devices while buses provide internal communication channels between components.