• The three main parts are the CPU, RAM and hardware controllerscomputer image by Ewe Degiampietro from Fotolia.com
  
  A computer is composed on many interrelated and interconnected parts, but the three main parts are: a central processing unit, primary memory, and input/output hardware controllers that control and manage all the peripheral devices such as the keyboard and monitor. Together these three main components operate over data taken in from the external world and then communicates the results of those operations back to the computer user.
  
  

CPU

• CPU stands for "central processing unit," and it is the part of the computer that controls and coordinates the activities of all the other parts. The CPU does this by running programs that use the computer in specific ways, such as sending data to the hard drive, printing a file, playing a video game, or helping the user pick stocks using the latest financial and stock analysis programs.
  
  The CPU itself is composed of three components: an arithmetic unit that handles mathematical computations; a programming "control unit" that reads and interprets a computer program; and an instruction decoding unit that converts the semantic language of a computer program written by a computer programmer into assembly language, the basic language understood by all the information-processing hardware components of a computer.
  
  

Memory

• After the instruction decoding unit converts a computer program instruction into an assembly code, it then stores the code in the computer's primary random access memory, or RAM. The assembly code stored in RAM is either an instruction, telling the computer what it needs to do next, or it is data. The instructions stored in memory tells the CPU how to operate over the data stored in memory, then the results of that operation are stored back in memory. For example, the computer may instruct the computer to add two numbers and then divide by a third number. The first operation would be to add the two numbers, and the second operation would be to divide the result of the first operation by the third number stored in memory.
  
  

Hardware Controllers

• Input units are peripheral devices that input data into the computer, such as an Ethernet card, a keyboard or a hard drive. The computer has separate hardware components called controllers that bring the data either into memory or into the CPU. Output units are those peripheral devices that communicate information to the user. These include a computer screen, a printer or speakers. These devices have hardware controllers that transform raw computer data into a format appropriate for each type of output device.