Evolution is a complex theory that has its roots embedded deep in to history. It is most often connected with Charles Darwin who wrote The Origins of the Species which was published in 1859. Most of Darwin's ideas weren't new ones, however this was the first time that these thoughts and ideas were published in a notable form. Over time the theory of evolution has evolved, with it ideas have been added, and this theory has been excepted by most as fact.
Darwin was a naturalist and as such spent a great deal of time observing animals and nature. He formed his theory of evolution from those concepts. It was his idea that at one time there was no life on Earth. Instead there was a pond and certain microscopic elements of that pond grew into a single celled organism due to nutrients and sunlight. From that single cell, Darwin proposed that all of life arose. He observed similarities among sub-species of animals and proposed that these similarities were a result of natural selection. Over time animals adapted to their environments and bred with the most desirable of their species producing stronger variations of themselves and over time changing their species. Darwin felt that this was the path that animals took from being a single celled organism to all the creatures found today.
Natural selection is what Darwin used to connect evolution. He figured that over time small micro-evolutionary events brought about the massive differences that are seen today. Natural selection or micro-evolution has been seen and documented by scientists. It is in fact, the only portion of evolution that has been proved. However, in all the cases of documented natural selection, there has never been a species change into another species before.
One simple example of natural selection can be given with a group of small lizards. There are with in this group small ones, normal sized ones, and larger ones. Because the larger ones are more likely to survive, they become the ones everyone wants to mate with. There fore their genes get passed down often. In addition to this the smaller ones and the normal ones are less likely to survive. Over time the lizard population becomes larger with natural selection.
Mutations are now included as part of the evolutionary theory. It is believed that the mutations of genes have helped speed the process of evolution. Through helpful mutations we have gone from single celled organisms to the complex variety of life seen today. These mutations are large genetic changes that can push a species from one type to another. There has been no evidence that such mutations exist or have happened in the past, but without them evolution would take much more time then it has.
Genetic drift is a newer idea and rather far from Darwin's The Origin of the Species. It is the idea that genes will show up more in populations that are separated from one another therefore taking populations in different directions. This would show how animals on one content or separated by a mountain range could vary from those found elsewhere.
While these theories may be far from the truth or right on, evolution has been an excepted theory without much proof. These explanations for how life evolved still don't answer some more basic questions about how it all got started and why we don't continue to see evolution happening before our very eyes.