How to define a Fitness Function in a Genetic Algorithm?

In my previous article, I have explained the basics about Genetic Algorithms. After it was published, I got many requests to discuss more about the Fitness Function and Evaluation Strategies. In this article, we will discuss about fitness functions and how to come up with a fitness function for a given problem. Fitness Function (also known as the Evaluation Function) evaluates how close a given solution is to the optimum solution of the desired problem. It determines how fit a solution is.