As an ex-photographer you should know what fresnel is (or at least a fresnel lamp), the relation between the fresnel lamps and fresnel shaders are pretty simple although I had no idea what they were at first. Basically, this Fresnel guy came up with some weird refraction calculations/laws. They were used when making the glass lens for the spots which are now known as fresnels to give a nice soft light. But they can also be used to calculate how transparent, or how reflective a surface is according to the angle between the light and the viewer. Baking means that a shader or part of a shader is converted into a texture map (going from procedural to image, or how the light hits it can be calculated into an image, and so on).