grounding is a mysterious art mastered by few. i don't understand it too well myself (i have no real background in electronics though), even after attentively following many posts about it here. if you really want a thorough and complete theoretical explanation, you're better off finding a book, i had a hard time finding good info myself on the internet.
a star ground i think is not the optimal way of grounding, but it is by far the simplest and easiest way of obtaining a functional, safe hum-free setup. Because of the simplicity of a gainclone, star ground is the obvious complement to it. i imagine in more complex systems, a star ground isnt even possible.
the idea of a star ground is that all your grounds join up into a final point. this inherently eliminates all possible ground loops. why? well because there is only one possible path to ground for every component. if you had uninsulated RCA's, you would create a second path to ground, and probably a nasty hum. It's a simple idea and it works.
other options are bus grounds for example. but i don't know much about that except they are harder to get right.
if it makes you feel better, most commerical equipment out there violates many grounding rules and is far from optimal.
