• Input. Accepting data that has been represented in a way that the computer can use
• Processing. Perfoming arithmetic or comparing (Logical) operations on the represented data
• Output. Displaying results
• Storage. Storing results for later reuse

A computer program is a list of instructions, written in a programming language, that tells the computer how to perfom these four simple operations in order to accomplish a task.

Computers are complex electronic devices, but their four basic operations--input, processing, storage, and output-are simple. Almost all computer processing, for example, involves simple operations such as adding two numbers or comparing two numbers to see which is larger. What makes today's computers so valuable is that the're fast and incredibly reliable. Even the least expensive personal computer can perform several million operations per second, and can do so for years without making an error caused by the computer's physical components. (Almost all "computer errors" are actually caused by flaws in computer programs or mistakes in the data people supply to computers.)