Out of the help files for a sampler info node

"Provides you with information about each point on a surface as it is being sampled, or calculated, for rendering purposes. Sampler Info can give you information about a point’s position in space, its orientation and tangency, and its location relative to the camera."

If you Plug the facing ratio into a ramp then play with the colours on the ramp thats then attached to a material it adjusts the colour (if that what you want) so that you see more (of less depending on how you set the ramp up) of the colour as the material "bends away" from the camera.

It sets up a fresnell effect, like when you walk past a building made of glass, as your walking down the street it looks totaly reflective but if you look straight as the glass its transparant, it sets that up usingthe cameera as your eyes so to speak.

Dust kind of works the same as you look at dust from an angle you can see it, as straight on it looks normal (depending on how dusty it is) if you use the incandesance channel of a material with a dusty colour or texture (defined on the ramp) you can get it looking like dust etc.

Hope this helps a bit.