Material which is not rendered but interacts with light It's hard to explain this. The way I'm visualising it is by thinking of an invisible object which reflects light. 



Let's say we this cube here with one face of it being lit up with a blue spotlight.

Now, let's treat the patch of blue light on the face of the cube as another discrete object (I understand it's physically impossible but bear with me).

I want to be able to keep that patch of blue light while making the cube disappear.

What I visualize in my head is a patch of blue light hovering.
My ultimate goal is to capture lighting information.

I hope this makes a little more sense.
In the object properties tab of the properties window, there is a section titled "Cycles Settings". If you uncheck the box "camera" under "Ray Visibility", that object will disappear from the render but still reflect light and cast shadows.