digital - What do all the settings do in Lightroom?

What do all the do all the develop settings mean in Lightroom?

For example, what does the Fill light setting do, and how does this differ from the Brightness setting?

I can of course just play around with the settings and see what the effect is, but I've found that I get better results when I stick to the settings that I understand the theory behind (such as Exposure and White Balance).