1. Computer System Unit Components

2. Motherboard (ROM)

3. The Motherboard is the central circuit board of your computer
The Motherboard is the central circuit board of your computer. All other components and peripherals plug into it, and the job of the motherboard is to relay information between them all. Despite the fact that a better motherboard will not add to the speed of your PC, it is none-the-less important to have one that is both stable and reliable, as its role is vital.

4. Power Supply

5. A power supply receives (240 in Europe) volts of AC power from a wall outlet and through the use of a switch mode power supply, converts it to 12 and 5V dc. The 12 Volts are used to drive most of the motors on the peripherals such as the hard drive mechanics and CDROM drive door and spindle, and the 5v is used to power the motherboard components. To support the CPU the 5v dc line is further converted to 3 volts to drive the CPU.

6. Drive Bay

7. The drive bay is an area of reserved space in a personal computer where hard or floppy disk drives, or tape drives, can be installed. The number of drive bays in a computer determines the total number of internal mass storage devices it can handle.

8. Central Processing Unit

9. The central processing unit (CPU) is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in use in the computer industry at least since the early 1960s. The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation remains much the same.

10. RAM

11. RAM (Random Access Memory) or main memory is where a computer stores programs, applications or data that is being used. Essentially the more RAM a computer has the more programs the computer can run simultaneously without showing a large slowdown in performance. RAM stores the data in memory cells that are arranged in grids much like the cells are arranged in a spreadsheet, from which data, in the binary form of 1's and 0's, can be accessed and transferred at random to the processor for processing by the system's software.

12. Hard Drive

13. A hard disk drive (HDD; also hard drive, hard disk, or disk drive) is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence "hard") rapidly rotating discs (often referred to as platters), coated with magnetic material and with magnetic heads arranged to write data to the surfaces and read it from them.

14. Fan

15. Fans are becoming more and more necessary for modern computers
Fans are becoming more and more necessary for modern computers.  They're used to simply move air through the computer or cool specific components.  What more people should realize is that not all fans are the same, and it is a wise choice to select high quality fans that last long, fit your noise tolerance and provide enough airflow for your needs.

16. Expansion Slots/Cards

17. A slot is an opening in a computer where you can insert a printed circuit board. Slots are often called expansion slots because they allow you to expand the capabilities of a computer. The boards you insert in expansion slots are called expansion boards or add-on boards. Do not confuse slots with bays. Bays are sites within the computer where you can install disk drives. Typically, slots are in the back of the computer and bays are in the front

18. Ports/Connectors

19. The various connectors and ports on the computer allow it to communicate with the many different devices and peripherals attached. Because there are so many cables and cords attached to the back of the computer, and so many different types of connectors, it often seems a little intimidating to the newer user. Although there are some devices which may use the same connector or port, the individual devices and their cords can only physically attach to one certain type of connector.