well a low grit compound would have a larger i guess pieces of sand (if thats what they are) but not as many. And a high grit compound would have many more pieces of sand but they would be much smaller. Its the same sort of thing as sand paper. the higher the grit the smoother the end result is going to be. but you always start out with something with a low grit to get the big stuff done then progressively move higher and higher in grit.
for the rubbing compound I would recomend 3M. If its not easy enough to order online just ask the guy who detailed your car who he gets his supplies from and he will either have it or be able to get it. its not available in stores either. and he will be able to tell you what you need also.
http://www.syntema.co.uk/3m_polishing_system_2.htmthat is what the stuff is. you will need two bottles of the 3M stuff. one high grit one low.
as for the wax...i cant remember the name. i will find out and let you know