What best describes Darwin's theory of evolution?

In simple terms, two things.

One, that if a species is placed in a situation where it must either adapt or die, only the strongest of the species will adapt and survive. The weaker of the species will die. This also applies to groups of species in competition - the stronger will live, and the weaker will die, hence, "natural selection."

Two, that, based on the natural selection and natural adaptation principles above, humans could not have been the way they are from the beginning of life on Earth. They evolved from another species, being an end result of a prior species' adaptation to their situation.

(For the record, and for any who stumble upon this, Darwin NEVER said that we evolved from chimpanzees. He stated that humans have a common ancestor with those lovable - but dangerous - monkeys. And, as any well-rounded scientist should, he never stated that the above was fact, hence why it is a THEORY of evolution and not a LAW of evolution.)