Lets start off with the sizes.
1/200 - the HCM scale. smaller figures, but with quite some detail.
1/144 - standard scale. most models come in this scale. has 2 grades, to be explained below.
1/100 - another well-known scale. comes in 3 grades.
1/60 - big ass models. 2 grades.
the grades:
no grade: this grade appears on 1/144 and 1/100 models. its the standard model, with standard detail. you get what you pay for.
high grade: this grade also appears on 1/144, 1/100 and 1/60 models. more detail, more parts, and often alot of extra`s. the high grade (HG) models` boxes are labled with a nice "High Grade" logo. the 1/60 scale regards this grade as "Big Scale" such as the 1/60 Freedom and 1/60 Impulse. basically the same model as the 1/144, but bigger and with some of the extra`s the bigger scale allows for.
master grade: this one appears on 1/100 models. a LOT of detail, and mucho parts. some include metal parts, others include a stand. nice models overall. this grade has a golden "Master Grade" logo on their boxes.
perfect grade: the creme de la creme. horribly expensive and an enormous ammount of parts. this is as close to the real thing as you can get. uberly detail, as accurate as possible. bandai has gone all-out on PG`s.