The input units, as the name implies, pass input information to the computers. The input information may be programs or program related, or it may be data.
Output units send the information from inside the computer to the outside so we can use it. The output from the computer may appear on a video display be printed out on a printer, be sent to another digital system over a transmission line, or be stored in auxiliary storage such as magnetic disks, CD, DVD etc.
The central processing unit, or processor, as some call it, is the nerve centre of the system. It performs the central control functions. All the computational, logical, and operational decisions are made here. It contains the logic circuitry for performing the various computational activities. It controls the operation of all the functional units. It fetches machine instructions from memory, decodes these instructions and insures that the operations called for by the instructions are executed correctly. In order to do all this, it communicates or interfaces with the input and output units and the memory.
The memory unit is used to store instructions and. Instructions in the form of digital codes are stored in a step-by-step sequence so the computer system knows what to do. While it is doing a particular operation it needs data to use or operate on. The data is usually brought in through input units and stored in memory. After the computer system has completed and operation it stores the results of the operation in memory. This also is data. Data may be words, letters, characters, numbers, symbols etc.