Evolution is the idea that living things change across generations, and that all of them descend, with modification, from shared ancestors. Charles Darwin called it "descent with modification," and it is the thread that ties the whole of biology together.
The core claim
Two claims sit at the heart of evolution. The first is common ancestry: trace any two species back far enough and their family lines meet. Humans and chimpanzees share an ancestor a few million years back; humans and oak trees share one far deeper in time, but they share one. All life forms a single, branching family tree.
The second is change over time: populations are not fixed. From generation to generation the mix of traits in a population shifts, and over long spans those shifts accumulate into dramatic transformation: fish lineages giving rise to amphibians, small dinosaurs to birds.
What evolution is not
It helps to clear away common misreadings. Evolution does not act on individuals within their lifetime: you do not evolve; populations evolve, across generations. It has no goal or ladder of progress toward "higher" beings; it is a branching bush, not a march. And "just a theory" misjudges the word: in science a theory is a well-tested explanatory framework, and evolution is among the best-supported in all of science.
Why it matters
Evolution is the organising principle of biology: as the geneticist Theodosius Dobzhansky put it, "nothing in biology makes sense except in the light of evolution." It explains why species are related, why organisms fit their environments, why life shows both endless variety and deep underlying unity. It also has urgent practical force: antibiotic-resistant bacteria, evolving viruses, and pesticide-resistant pests are evolution happening now, on timescales that affect medicine and agriculture directly. The lessons ahead build the mechanism piece by piece: where variation comes from, how selection acts on it, how it builds adaptation, and how it splits one species into many.