L is a scale from white to black, much like the Value in the HSV (Hue, Saturation, Value) model

A ranges from green (-a) to red (+a)

B ranges from blue (-b) to yellow (+b)

The advantages of using L*a*b for color coding are basically device-independence and the fact that brightness has its own axis (the L axis), so that you don’t have to deduce it from the other real color-related values (as is the case in RGB, for example).

All colors in RGB can be displayed as a color diagram called the gamut. Since this gamut represents all possible colors (only the ones CIE Lab, actually, but the range is really big enough for human beings), other more limited colorspaces like RGB and CMYK can be represented as a part of the gamut space. RGB, for instance, forms a triangular shape in which red, green and blue form the corners, while CMYK is a much more circular form which allows for more shades of colors other than red, green and blue.

All colors in CIE Lab can be displayed as a color diagram called the gamut.

and hurrah for colourspace finally working in the gimp; now if only it had dynamic size layers!