Mysterious Nodes

So, I’ve been trying to get better at node-based materials and texturing lately, but there are a several nodes I simply don’t understand completely.

They are the following:
Geometry (Both the BI and Cycles versions)
Light Path
Fresnel and Layer Weight: why have both? (Cycles only)
Camera Data
Texture Coordinate

Anyone who can explain and help me understand any or all of these gets my thanks and imaginary cookies for awesomeness. :yes: