Reading (or tasting) it is easy to find dozens of different styles, going a litte more in details than lager/ale distinction, what is a simple categorization of beer styles?
Like fire.eagle said, I think the BJCP style guideline is what you are looking for. This is what nearly all home brew competitions use when judging beer.
You can also look at the Great American Beer Festival's style guidelines. They break beers out into more categories than the BJCP.