Computer programs can be written to carry out a wide range of tasks, from writing letters to guiding a space rocket. Programs are particularly good for repetitive tasks, since computers can work very fast; they do not become bored or tired and they can do tasks involving calculations with 100 per cent accuracy. However, computers have no in-built intelligence and are incapable of independent thinking- they can only follow the instructions within a program.
Whatever function you want a computer to perform, all you have to do is write the program. A program is a little like a recipe for cooking a meal. A recipe lists, in detail, the steps you must follow to make the meal. Recipes are written in English, to be understood by humans and assume a level of common sense from the cook.
Computers, on the other hand, do not have common sense and require a very precise set of instructions.