A theory of evolution is an attempt to explain how evolution occurs. There are countless theories of evolution, many of which are compatible with one another. Others, such as Lamarck's theory of inheritance of acquired characteristics, have been falsified.
The commonly accepted scientific theory today is called the Neo-Darwinian Synthesis, based primarily on Charles Darwin's the theory of natural selection, but updated with newer discoveries in biology and genetics. In popular usage, "the" theory of evolution refers to this or other Darwinian theories.
Important concepts in evolution: