Here's a breakdown:

* Evolution: The idea that life on Earth has changed over time, with new species arising and others becoming extinct.

* Natural Selection: The mechanism by which evolution occurs. It states that organisms with traits better suited to their environment are more likely to survive, reproduce, and pass on those advantageous traits to their offspring. This process leads to a gradual change in the characteristics of a population over generations.

Darwin's theory, laid out in his groundbreaking book "On the Origin of Species," revolutionized our understanding of the natural world. It challenged the prevailing view of creationism and provided a scientific explanation for the diversity of life on Earth.