What is the theory of evolution?

Although referred to as a theory it is in fact proved (creationists just won't accept this pure science!)
Life began as a simple organism, as time went by it exploited opportunities provided changes in it's environment and diet. It did this by changing it's nature to more and more complex and successful forms. Any change to the environment and diet will inevitably led to a change in the organism that again exploited the resources available. Less successful organisms (or organisms that don't change with it's environment) are superseded and die off, hence the term "survival of the fittest".