Evolution

Not to be confused with "Evilution".

While the usage of the term can differ, in this context evolution refers to the process of changes in the inherited traits of reproductive populations over generations, which is summarily defined as "descent with [inherent genetic] modification" as coined by Charles Darwin. It is a natural phenomenon that encompasses all life, and more.

The theory of evolution is the scientific model that explains how and why evolution occurs, as well as the unity and diversity of life (not its origins) by concluding that every known species extant or extinct, is the result of evolution through common descent (and a few newly discovered processes such as "horizontal gene transfer" and "endosymbiosis"). As a result of the overwhelming evidence in favor of the theory, it represents the consensus of the scientists in the field of biology (and science in general), thereby making it one of the most well-supported theories in science. The theory of evolution is the very backbone of modern biology and understanding evolution has become a fundamental aspect in that particular field of science.

Understanding evolution (especially how it works) isn't easy. In order to adequately understand evolution, you also need to have a basic understanding of many different things, including cellular biology, genetics, anatomy, geology, paleontology, taxonomy, and more. It is a very complex subject and passing biology classes in college requires following many lessons on evolution, depending on what schools you are talking about. Many notable scientists throughout history have devoted their entire careers on this very subject, and many still do. Despite this, there are many common people (often those without an education) who dismiss the entire scientific paradigm by asserting that evolution is "just a theory, not a fact", as if they know better than every expert anywhere ever.