Egads! Back to school for me: Lab color space is basically, a number for Lightness (L - 0 for darkest black and 100 for bright white) and 2 more for color combinations (a and b). It includes all perceivable colors (RGB and CMYK for example) and is designed to approximate human vision. It is designed to be device independent and it's used to convert between color spaces (for example when converting from RGB to CMYK and vice versa) in many software apps, RIPs, and graphics drivers. In photoshop, you can use it for Photoshop files, but for output, you will need to convert to RGB or CMYK or another profile. My understanding is that it is mostly used as reference since most image editors use it in the background and, while you can use Lab for your project, many filters and actions aren't usable in Lab mode.