the maya node system is basically a collection of different items (nodes) that you can link together, say for example if you wanted a bump map on a texture then you would first of all create the relevent material (i.e. a blinn or lambert etc) and then link the bump value to a texture. The power of maya's hypergraph lies in the fact that you can link almost any attribute to any texture/utility/shader etc. once you get into it you'll be fine.

about mental ray, i don't use it at all, i've found a way of getting nearly the same quality without the long render times. (BUT IT'S A SECRET) lol