The fill light slider brightens and recovers detail from the shadows using an HDR style tonemapping operation. Used in moderation it can increase the apparent dynamic range and deal with large distracting overexposed areas. Turned up to 11 you will get a nasty overprocessed bright-everywhere image that will make people on Flickr go "wow great capture".

The clarity slider does something akin to high pass filtering, basically increasing the contrast of large details without adding noise. Used in moderation it improves the, well, clarity of the image, especially when there is low contrast or flare. Also looks good on clouds. When overused it makes everything look a bit plasticy and fake.

The vibrance slider selectively boosts the less saturated colours. This increases the overall colourfulness without pushing existing colours too far (which can happen when you increase saturation). Again if used too much you'll get something that is painful to look at. You can combine increasing vibrance with decreasing saturation which evens out the colours even more, I find this helpful to boost blues and greens whilst keeping skintones in check.

These are the main ones that are tricky in their operation, perhaps you could be a bit more specific about which settings you'd like to understand better.