The essential components of a PC are the mainboard, processor and memory. The memory stores the computer program the processor is running and tile data the program needs. The mainboard is the printed circuit board that the processor and memory is mounted on. It connects the processor and memory together. Let's look at these in more detail. Remember, a computer can only work with numbers. The processor does all the calculations with the numbers, obeying the instructions in the computer program. The memory consists of tiny circuits called a memory cell. Each circuit can store a zero or a one. To make a byte of memory, you need eight of these circuits. So when you buy a 128 Meg memory strip (128 million bytes of memory), it contains 128,000, 000×8 memory cells that is, 1, 000, 000, 000 memory cells. Yet a 128 Meg memory strip is not much bigger than your finger!
A memory strip actually contains other circuitry as well. This circuitry allows the processor to select which memory cell it wants to get data from, or to store data into. The processor can jump directly from the first memory cell to the last, then to a cell in the middle, or to anywhere else to get or store data. That's why this kind of computer memory is called Random Access Memory, or RAM for short. RAM needs electricity to be able to remember data. When you switch off your computer, all the data stored in RAM is lost. For this reason, computers need some kind of permanent storage. That's why computers have disk drives. Computer people call disk drives, magnetic tape, CDs, DVDs and so on secondary storage.
The mainboard connects all the components of the computer together. It has electrically conducting tracks which allows electricity to flow from one component to another. For example,from the processor to the RAM.
A computer takes some kind of input, processes it in some way, then outputs the result. For example, you scan a picture into your PC. This is the input. You then modify the graphic — you adjust the color balance, the contrast and the brightness. This is the processing. Finally, you print the graphic. This is the output.