Global lighting effects the intensity of the lights you have in your scene. By changing the global and ambient values you can make you scene darker or lighter. I belief the ambient chauses to the scene to be darker, but i'm not sure.
How do you do ambient
I think you mean the AmbientOccluder attribute? This is all explained in an other topic created by steve. Short said you go into scene mode, creat a new attribute called 'AmbientOccluder'. then you make some in changes in the global lighting setup: uncheck project settings and make the values global:0 and ambient:3.14. Then render.
RayDepth attribute sets max depth rays are traced in the ART ray tracer.
I hope this answers you questions a bit.
