The theory of evolution proposes that individual life forms inherit traits from their parents and pass those traits on to their offspring, and that because some individuals survive to reproduce and others do not, that the ones that do survive will pass on more of their traits into future generations, thus causing the frequency of different traits in any particular generation to change over time.

There are several different processes of evolution concerning how these traits get passed on and why some are passed on more than others. The theory of natural selection states that some traits actually help the life forms that have them, and thus cause themselves to be passed on more often, becoming more common in future generations. Genetic drift is when some individuals happen to pass on their traits and others to not do so by sheer luck, with there being nothing to do with how good or not good the traits are. Mutations are the source of new traits. All of these are subject to the environment; for example, if climate changes, then which traits are good for natural selection and which traits are not so good may change in response to climate (a thick coat of fur might not be needed to keep you warm in a hot desert like southern California, but it's really useful in a colder climate).

All four of these processes have been seen at work in nature. The unanswered questions regarding the theory of evolution mostly have to do with how important each of these forces is in shaping the history of the world that we can see in fossils, DNA sequences of living animals and plants, etc. At this point, the theory of evolution becomes a great deal more complicated, and the mysteries are still numerous. Darwin, for example, when he formulated the theory of natural selection during the first half of the 1800s, thought that almost all extinction was due to natural selection and specifically due to competition between superior and inferior traits. We don't think that's the case anymore; the majority of extinction is more likely due to climate change, but we still don't know for certain.