The way I remember it is simply that A is the highest controlled form, and G is not controlled at all. Everything else is somewhere in between and the details come from that:
A-E officially called "controlled", F-G officially called "uncontrolled"
A-D needs clearance to enter (A is actually IFR only, with a very few cases of SVFR occasionally granted), E-G no clearance required but radio contact with the controlling agency recommended.
A-E gives traffic information & separation in ever decreasing amounts for IFR and VFR. F-G no traffic information by default, although in some cases you can get a RIS/LARS.
VMC weather minima are the highest in C-E. Lower minima in B because everything is controlled anyway so less requirement for see and avoid, and lower minima in F-G because it's not busy (yeah right). The minima go up above a certain FL because people are allowed to fly faster there. I used to know the exact numbers for my exam, but forgot and put them on a sticker on my clipboard in case I ever need them.
Not all countries use all classes but since I fly on the continent with a CAA/JAA PPL, I thought it best to remember them all.