Google Explains, How Search Works!
A few times I have looked at explaining just How Search Engines Works, but I feel I've been out-done by Google. Google recently released this awesome interactive infographic, it means you can learn some great facts about search and how it works.
You can find the infographic @ http://www.google.com/insidesearch/tipstricks/
Once you are content with how Search works, take a look at the policies page - it's got handy info on how things work in the background, such as why algorithms are favored over manual rankings and just how spam websites are fought on a large scale.