The system unit is basically the whole computer minus the monitor and peripherals like keyboards, mice, speakers, webcams etc.
Most system units will be composed of:
Usually referred to as the CPU, it's the brain of the computer and its most important part as it performs the instructions of computer programs and executes the functions of the computer.
Most commonly a CD and/or DVD disk drive nowadays, though there are still computers in operation with floppy disk drives, they are used to read externally inserted disks so as to extract data off them.
These include graphics cards and sound cards that offer new functions and possibilities to the computer and its users. Most units will have spaces for these cards to be easily added.
Otherwise known as the hard drive, this is a computer's main storage component, this is often confused with memory, or RAM.
Commonly known as RAM, it supplies and holds information that the CPU uses while the computer is switched on, but does not store it to permanent memory.
Like a battery, no juice no work! The PSU in most PCs converts the AC electricity from the mains into DC to eliminate surges that may damage the delicate infrastructure of the system unit.
Otherwise known as a display adapter, they are used as a medium between the system unit and the monitor.
