Charles Robert Darwin (1809-1882) was a geologist, naturalist, geographer, and biologist. He did some groundbreaking and very, very extensive studies on earthworms and barnacles, but he is best known as being the one who, along with Alfred Russel Wallace, first proposed the theory that new species could come about through natural selection.
At this time, any theory of evolution was quite controversial, as it challenged the standard interpretation of God's role as the creator of everything. God was supposed to have had created the world, and all the animals in it, just as it is, with no changes since the initial creation. The idea that new species could pop up was scandalous.
Darwin had a rather uneventful youth, playing and hunting in the country side. When he went to the university Darwin studied medicine because his father wanted him to, but he found that he didn't have the stomach to operate on people without the benefit of anesthetics. Next he tried theology, his father's second choice. This worked out okay, but one of his professors, John Stephens Henslow, got Darwin interested in natural science. This was clearly what he wanted to do with his life, and Darwin never looked back.
By a tremendous stroke of luck, Darwin got a job as the ship's naturalist under captain Robert FitzRoy. From 1831-36 he sailed aboard the HMS Beagle, exploring foreign lands and collecting hundreds of samples of odd forms of life. It is likely that FitzRoy took on Darwin because he wanted someone he could talk with on the long voyage, as FitzRoy couldn't converse with his subordinates (the idea!). Darwin was accepted in part because he was a gentleman, and probably because he had a background in theology (FitzRoy was quite religious). Darwin was also the first naturalist FitzRoy found who didn't mind paying his own way, which probably was the deciding factor. They had some spats over slavery, which Darwin was against and FitzRoy was for. The primary topic of discussion was probably religion.
On this voyage Darwin made observations in South America and the Galapagos Islands that later were incorporated into 'On The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life' in 1859. At this time, however, he was thinking in terms of evolution.
Once he was back in England -- and it is worth noting that he never left England again -- Darwin set about cataloging his samples. About this time, he came across a book, Essay on the Principle of Population by Thomas Malthus, which pointed out that populations grow faster than their food supplies, meaning that death was a constant and unavoidable part of life. This apparently triggered Darwin insight into the workings of natural selection. By 1842 Darwin was putting his ideas into writing -- but not sharing these writings with anyone else.
Charles Darwin also did extensive research on geology, barnacles, the fertilization of plants, the construction of coral reefs, and earthworms. In fact, part of the delay betwixt the voyage of the Beagle and the publication of Origin of the Species was eight years full of barnacles. (Darwin was, and to a large extent still is, one of the most respected authorities on barnacles).
In 1858 a friend of Darwin's, Alfred Russel Wallace, sent Darwin a letter, and then paper, explaining the idea of natural selection. Darwin let Wallace know that he had been working on the idea for 20 years, and was almost ready to publish. When Wallace's paper arrived, Darwin (at his friends urgings) had both his paper and Wallace's read at a meeting of the Linnean Society, and published together in the society journal. (Darwin was not at the reading, as one of his sons had just died of scarlet fever.) Wallace was still overseas, and at the time had no idea that anyone was planning on making his paper known. In the presentation it was made clear that Darwin was the first to come up with the idea. Darwin felt bad about the whole thing, but Wallace forgave him.
The following years were full of violent (well, volatile, anyway) disputes over evolution, natural selection, and the origin of species. Darwin avoided it as much as possible, allowing others to defend his idea in public. While Darwin was a firm believer in change through natural selection, he did see some problems with it. (How could evolution produce something as complex as an eye? Wouldn't evolution through this method require literally millions of years? (gasp!) And perhaps worst of all, Darwin's wife was very religious, and he didn't like to upset her.)
Darwin's theory of evolution has been in dispute ever since, but Darwin did live to see his theories become somewhat accepted, and certainly quite respected by many.
*Darwin came up with the idea of natural selection before Wallace, but didn't publish it until he saw that Wallace was onto the same idea.
Patrick Matthew also did some work in this area before either Darwin or Wallace.
Darwinism cannot explain all evolution, and thus Neodarwinism is now used to fill in the holes.