Well for nvidia, the first number is the series or generation if you prefer.
The second number is the relative performance in that series, for example
x1x, x2x, x3x, x4x is low end card (where the first x is there series it came from)
x5x, x6x is midrange
x7x, x8x, x9x are the high end
The 3rd number is the little increments for example a GT 555m is a bit faster than the GT 550m.
Nvidia tried to make it easier for the average consumer by using the GT and GTX, which I think just makes it more confusing. GTX is faster or should i say fastest, lowest is GS as I recall.
The "m" just stands for mobile which makes it less likely for people to get it confused with the desktop versions with the same names.
The desktop versions are faster.