Darwin Day 2013
The basic idea that simpler life forms have gradually developed into more complex ones over time dates back at least to the Romans. Darwin's theoretical contribution was to propose the mechanism by which this process happens. Any population of organisms contains some variation among its members, and some of these variations will be more conducive to survival and reproduction than others. As a result, the individuals which carry those variations will usually produce more offspring, and if those favorable variants are inherited traits (an important qualification), they will become more common in the population, so that over time the nature of the population changes.
It seems like a tiny effect, but operating over long enough spans of time, it can change a population of simple self-replicating molecules into a population of highly complex organisms such as you and me.
(Note that while the mutations that produce the basic variations are random, natural selection is not random. When you hear someone attack evolution as a "random" process, this is a sure sign that he lacks even rudimentary knowledge about it.)
Nor did Darwin stop at just proposing the idea. By his exhaustive study of animals he was able to assemble a great mass of supporting evidence -- the real meat and muscle of science. It's not enough to propose an idea that "rings true" or seems intuitively convincing. The real test is always the evidence supporting the idea, and how well that evidence stands up to challenges.
Darwin did not set out to attack religion, but inevitably his theory was one of the mightiest blows against religion ever delivered. However much of religion's specific dogma had been exposed as nonsense down through the centuries, the complexity, sophistication, and apparent purposeful design of living organisms had always been an unanswerable argument for the existence of some kind of guiding intelligence at work in the universe. By providing a real explanation for those phenomena, Darwin swept away this last seeming rational basis for belief in a supernatural creator.
The theory of evolution as we have it today is far more elaborate and sophisticated than what Darwin originally proposed, because we have learned far more in the meantime (genes and DNA were not discovered until long after Darwin's death, for example). This is how science works. It's only worthless belief systems such as religions that operate on the model of a one-time Revelation from the Master, immutably set in stone, to be "interpreted" but never improved or changed by the lesser men of later generations. Science is always seeking new data and testing its theories against those data to arrive at a more accurate understanding of reality. But the present-day theory of evolution is still rooted in the concept of gradual population change via natural selection which Darwin originated. And so massive is the amount of supporting evidence discovered since Darwin's time, that evolution now ranks among the most solidly-established facts in all of human knowledge -- for more on this, I recommend Richard Dawkins's numerous books on the subject.
Darwin's Wikipedia article
Darwin Day Foundation
Reminder for modern conservatives