Although it is often overlooked, Lab colour is an extremely useful colour space in Photoshop that enables you to separate luminance information from colour information.
The more familiar RGB colour space achieves colour and brightness through mixing channels of red, green and blue light. By mixing the three channels, each with its own level of colour and saturation, an overall range of tones and colours is achieved.
Conversely, Lab splits all the colour information into two channels, but separates the lightness, or luminance, onto a third independent channel. This difference in channel make-up enables some eye-popping colour treatments and corrections that just aren't possible in RGB, all without compromising the luminance, contrast or detail of the image.