These websites provide good starting points for research on evolution, from basic explanations of evolution itself to more in-depth resources about different facets of evolution.
Evolution, simply put, is descent with modification. All living things on Earth are descended from a common ancestor, and the vast differences between species are a result of many modifications over the last few billion years.
Several general or popular works on evolution for beginners or for the curious.
On the Origin of Species by 
Darwin for Beginners by 
The Greatest Show on Earth by 
The Beak of the Finch by 
Undeniable: Evolution and the Science of Creation by