Charles Darwin developed the Theory of Evolution by natural selection in his 1859 book The Origin of the Species. He argued that all life is descended from a common ancestor and that living organisms adapt to their environment to increase their chances of survival. This is not something that occurs quickly, but over generations of a species and millions of years.
One of the important principles of the theory is natural selection. Natural selection is the idea that organisms best adapted to the environment will survive long enough to reproduce and carry on that specific trait. For instance, tree frogs have the ability to camouflage themselves to hide from predators. Another example is rabbits with white fur in cold, snowy climates. While camouflage is arguably the most recognizable trait with natural selection, there are others such as speed, hibernation and scent. All of these traits help a living organism survive.
Another key phrase you'll hear is, "survival of the fittest." It is important to note that "fittest" does not mean the most in shape or strongest animal. Rather, it means which animal is best adapted to the local environment. So, the phrase essentially means those that have traits best suitable to their surroundings will survive. This carries on through the generations of that organism until that trait becomes permanent.