Charles Darwin is the author of the Theory of Natural Selection, an explanation of the process of evolution, and not the "Theory of Evolution", which is a misnomer. Indeed, the concept of evolution has been around since even before the days of Aristotle. After graduating from Cambridge in 1831, Darwin took up a job as a naturalist aboard the HMS Beagle. It was this job that gave Darwin the chance to observe many natural forces around the world. He noted the similarity between many living species to those that had been extinct and fossilized. While on the Galapagos Islands, he noted that each island had its own form of mockingbird, tortoise, and finch. They had different structures and eating habits from island to island.
So Darwin began to postulate on the changeability of species. By 1838 he had developed a rough outline for his Theory of Natural Selection. It was first presented in 1858, and then published in 1859. This theory postulates that species must compete to survive. The young that survive to produce a next generation inheirit favorable variations, however slight they may be. Those who don't, don't. These variations are passed on genetically, allowing for each generation to adapt. Darwin also introduced the idea that all similar organisms came from common ancestors.