Key Light: The main light (the one that provides most of the lighting and shadows.
Fill-in Light: Used to provide (usually low-level) additional light in areas that are too dark. (often with no shadows, although you can't do that in photography...)
Back Light: A  (usually low-level) light that is behind a figure, to provide a a bit more modelling...
Hope that helps:-)

Also, google for "Art Theory" and "Art Composition", to get tuts on where to place figures/props in relation to each other and why...

I have a couple of tuts on my HD, but can't remember where I got them from, so can't give links:-(

Cheers,
Diolma