The CIE L*a*b* colour model
Most photographers will seldom, if ever, use this colour model directly in regular image processing. However, its colours are the basis of all device independent spaces, and it is important in the process of producing accurate ICC device profiles.
The CIE L*a*b* system describes all CIE colours using 3 numbers: L, a, and b.
The a axis measures colour on a green to red/magenta continuum with -ve values being green and +ive values being magenta.
The b axis represents colours on a blue to yellow continuum.
The L axis represents 'Lightness'. An L value of 0 is black, while a value of 100 represents the brightest values. A value of L=100, with a and b both 0, should be white.
The pictures at the right, taken from Wikipedia article, shows how you can see all the colours in slices across the L axis.
The importance of the L*a*b* space is that equal steps in a, b, or L values represent roughly the same percieved step in tonality for a human observer. (That is not true in the RGB models where the same steps in the R,G, or B values might represent quite imperceptible changes in tonality in one part of the space but large changes in another part.)
This characterisitc makes the L*a*b* representation of colour very useful for editing colour values when building device profiles that will represent colours for human, as opposed to computer, consumption!