Input unit – This is the “receiving” section of the computer. It obtains information (data and computer programs) from input devices and places this information at the disposal of the other units for processing. Most information is entered into computers through keyboards and mouse devices. Information also can be entered in many other ways, including by speaking to your computer, scanning images, uplaoding digital photos and videos, and receiving information from a network, such as the Internet.
Output unit - This is the “shipping” section of the computer. It takes information that the computer has processed and places it on various output devices to make the information available for use outside the computer. Most information output from computers today is displayed on screens, printed on paper or used to control other devices. Computers also can output their information to networks, such as the Internet.
Memory unit - This is the rapid-access, relatively low-capacity “warehouse” section of the computer. It stores computer programs while they are being executed. It retains information that has been entered through the input unit, so that it will be immediately available for processing when needed. The memory unit also retains processed information until it can be placed on output devices by the output unit. Information in the memory unit is typically lost when the computer’s power is turned off. The memory unit is often called with memory or primary memory. [Historically, this unit has been called "core memory," but that term is fading from use today.]
Arithmetic and logic unit (ALU) – This is the “manufacturing” section of the computer. It is responsible for performing calculations, such as addition, subtraction, multiplication and division. It contains the decision mechanisms that allow the computer, for example, to compare two items from the memory unit to determine whether they are equal.
Central processing unit (CPU) - This is the computer’s “administrative” section. It coordinates and supervises the other sections’ operations. The CPU tells the input unit when information should be read into the memory unit, tell the ALU when information from the memory unit should be used in calculations and tells the output unit when to send information form the memory unit to certain output devices. Many of today’s computers have multiple CPUs and, hence, can perform many operations simultaneously—such computers are called multiprocessors.
Secondary storage unit - This is the computer’s long-term, high-capacity “warehousing” section. Programs or data not actively being used by the other units normally are placed on secondary storage devices, such as your hard drive, until they are needed, possibly hours, days, months or even years later. Information in secondary storage takes much longer to access than information in primary memory, but the cost per unit of secondary storage is much less than that of primary memory. Other secondary storage devices include CDs and DVDs, which can hold hundreds of millions and billions of characters, respectively.