What is an algorithm?

      This is a question posed by many people who do not study computer science. However, in this increasingly computer literate world, it would be more beneficial to know some algorithms as they impact your daily life through you using the computer. 
      An algorithm is, roughly, a description of how a certain task is done. Using this definition, it can be said that opening the door is the name of an algorithm as the completion of this algorithm would enable you to open the door! Indeed, from this viewpoint, it can be said that human lives consists of algorithms: Deciding what to eat, what to wear, what to do, all these are familiar examples of how algorithms exist in our daily life.

Take this algorithm for example:
      Algorithm OpenDoor
           Step 1: Raise your hand so that it rests on the doorknob.
           Step 2: Twist the knob left.
           Step 3: If the door opens, stop. If not, turn the knob right.
      End

      Of course, an algorithm has certain chracteristics that set it apart from just any procedure for doing things. Donald Knuth listed, in 1968, 4 chracteristics of an algorithm: 

      1) An algorithm always terminates. This means that an algorithm always stops, however long it takes to stop. Returning to our previous example of opening the door, you can see that the first criteria of the "open the door" being an algorithm has been met because the "open the door" algorithm will stop after you have opened the door. By this same definition, an algorithm that tries to calculate the exact value of pi is not an algorithm as it will never stop.

      2) In an algorithm, every step is described precisely and unambiguously. Thus, our "OpenDoor" algorithm is an algorithm as it describes what needs to be done precisely and without ambiguity.

      3) Every step of the algorithm is simple enough so that it can be done in a reasonable amount of time. Our "OpenDoor" algorithm also conforms to this statement as each of the steps can be accomplished within a reasonable amount of time. 

      4) Finally, an algorithm always produces an output, although an input may not be required.