Channels

From ZBrush Info

To begin, let’s look at the concept of “channels” within image data. A channel is just a set of data that all has to do with specifying the “same type” of information in the image, over the whole of the image. As an example, an image file format dealing only with grayscale images might have just one channel, and that channel would contain a grayscale intensity value for each point in the picture. On the other hand, most color image file formats define (at least) three channels; one contains the red intensity at each point in the picture, one contains the green intensity, and the third contains the blue intensity.

So an image file generally contains one or more channels of information, and each channel contains values affecting some particular aspect of the appearance of the points in the image. Red, green, and blue channels are the most common, but channels can be used for other things also.