Chrominance is often used when talking about the LAB colour scheme. LAB produces a full 16.7 million unique colours, and is comparable to the HSB or RGB schemes, but uses two independant 8-bit (or 16 bit depending on the final colour precision used) chrominance channels seperate from a third luminosity channel to define a single colour. Thus, there is chrominance A, (an 8 or 16-bit channel), chrominance B (another 8 or 16-bit channel), and luminosity, which dictates how bright it is. Chrominance is the term that is used to describe the components of an image in terms of colour alone, without the concept of light or luminance. This concept can be difficult to visualize in your head because colour is commonly recognized as a property or attribute of light, not seperate.
Anyone who is used to using photoshop for image manipulation should be familiar with chrominance, in order to more easily manipulate images by their colour components without worrying about the right brightness.