The grid system - or so I currently understand it - is essentially an extension of two mathematical concepts; The Golden Ratio and fibonacci sequences. Applying these rules and concepts to the idea of design has led to the "grid system" genre.

I am too pretty new to all this and find that the best way to understand a subject is to start from the beginning. This seems to be a pretty good article on the golden ratio (http://milan.milanovic.org/math/english/golden/golden1.html) have a read. From there I recommend looking into fibonacci sequences - there not as scary as they sound.

In theory the idea of grids can be applied to anything, a line that seems to keep cropping up in this area is "the grid is not an answer as such, it serves to aid creativity by providing a solid aesthetic base" - I paraphrase, but that's the gist of it.

Hope this helps.