HG : High Grade
High Grade are the standard gunplas which have mediocre attributes like color, detail, articulation, posability, etc.
HGUC : High Grade Universal Century
Basically this is also a HG, but the gunplas in this line only from the Universal Century timeline.
SD : Super Deformed
The minified version of Gundam, you can say that it is a chibi version and looks cuter than other grades.
MG : Master Grade
The gunplas in this grade are for modelers who are looking for better challenge than the HGs. The MGs usually have better color separation, better detail, better articulation, and more excitement because more runners in it. Well, some might fell that more runners is worse though. ^_^
PG : Perfect Grade
Enough said, Gundams in this grade by far have the most detailed components than the other grades. The PGs are expensive, very big (1/60 about 30cm tall), and the selections are quite minimum.
RG : Real Grade
The newest addition by Bandai, it provides us with the tremendous details in a 1/144 size gunpla. Palm sized that is.