http://zh.wikipedia.org/zh-hk/Lab%E8%89 ... A%E9%97%B4Usually instead of breaking it in to RGB it become L(lightness) and colour A and B. It react totally different from RGB. Now I read the thing in wikipedia and have no idea what it means. So I cannot explain it well to you.
Usually I use it for touching up photos to get a specific effects. When you apply a colour balance to a Lab colour image in photoshop. Notice that it gives you no more RGB to control. instead gives you Green, Megenta, Blue and Yellow. Or if you go into curves, select A, will be able to adjust Green/Megenta, select B will be able to adjust Blue and Yellow.
L is just the lightness of the whole image without touching the colour.
For adjusting the brighness of the image without touching the colour. You may switch to Lab by adjusting L(lightness) will not effect the colour of the image.