A computer has four main components: the central processing unit or CPU, the primary memory, input units and output units. A system bus connects all four components, passing and relaying information among them. This type of computer organization and architecture is called a "von Neumann machine" after John von Neumann, who finalized the theory and design of the first modern digital computer.
CPU
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Computer scientists typically call the CPU the "brain" of the computer because this is where programs are executed. A program is a set of instructions that tells the computer how to accomplish a specific task, such as sending a file to the printer, opening a browser window, or playing music or video.
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The CPU is further broken up into three smaller components: the arithmetic unit handles all the simple mathematical computations; the control units interpret the instructions in a computer program; and the instruction decoding unit converts computer programming instructions into machine code. Machine code is the basic language understood by all the components in a computer.
Memory
Once the CPU converts a specific set of computer program instructions into machine code, it stores that machine code in primary storage or memory. The machine code will be treated as either data or instructions. The CPU fetches data and instructions from memory, uses an instruction to manipulate the data, and then sends the result and the next set of instructions back to memory.
Input Units
Input units are all the devices you use to feed information to the computer, such as a keyboard, a hard drive or a networking card. These devices, in essence, bring data from the "outside world" into your computer, in much the same way that your eyes and ears bring information to your brain. Each input device has its own hardware controller that connects to the CPU and primary memory, and it has a set of instructions that tells the CPU how to use it.
Output Units
Output units are the devices your computer uses to relay information to the user, such as a printer, monitors and speakers. For example, everything you see on your computer monitor starts as machine code in memory. The CPU takes that machine code and converts it into a format required by your monitor's hardware. Your monitor's hardware then converts that information into different light intensities so that you see words or pictures.
The System Bus
The system bus lets the four components of the computer communicate with one another. The system bus transmits data and instructions. It also sends addresses that tell the CPU where in primary memory the data and instructions are coming from and where the results should go.