decipherinfosys posted a link to a really informative page that has an introduction to Oracle architecture. If you're familiar with other databases and have considered checking Oracle out, this might be right up your alley.
If you've been holding off on playing with it because you've heard that it's really expensive, well, yes, it is expensive, but the license states that you can download it and play with it for free, as long as it's "only for the purpose of developing, testing, prototyping and demonstrating your application". So yes, you can get experience playing with Oracle for free, you just can't put it into production.
So read, download, play, and enjoy!