There's the 200, 300, 400, 500, 600, and 700 series. The higher up you go, the better the parts and quality of the guitar. Anything 500 and above is made in Japan. 400 are Korea, 200 and 300 are made in China, I beleive. Anything 500 and above you get the top of the line tremolo and tuners (Edge, or these days, Edge pro, and gotoh tuners)
A few years ago they turned the japanese line into the Prestige line, so an RG 570 became the RG 1570, etc...
a "50", meaning 350, 450, 550, etc.. means it's a HSH with a pickguard.
a "70", meaning 370, 470, 570, etc... is a HSH with no pickguard.
a "20, such as 520, means it's HH.
As mentioned above, DX usually means it has sharktook inlays and binding.
A "T" anywhere in the name- ex. RGT3120, means it's a neck thru.
"FM" anywhere in the name means it has a flame maple veneer top
That's generally the designations. Then there's others, depending on the color, finish, years, etc....