Evolution
Not to be confused with "Evilution".
To MrIntelligentDesign, his comments has been moved to his own page, due to its lack of scientific accuracy and coherency.
Description
While the term is used in various ways, in this context "evolution" refers to change of inherited traits among 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 the diversity of all life. One of the driving mechanisms behind this process is "natural selection".
The theory of evolution (ToE) is the scientific model that explains how and why biological evolution occurs, as well as explaining 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 other 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 strongest and 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 need to have a basic understanding of many different things, including cellular biology, genetics, anatomy, geology, palaeontology, 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 to this very subject, and many still do. Despite this, there are quite a few people who dismiss the entire scientific paradigm by asserting that evolution is "just a theory, not a fact" (among other asinine statements), as if they know better than every expert anywhere ever. The bulk of this category consists mainly of scientifically illiterate morons.