Channels are another way of looking at color. Each image has one or more channels, with the number depending on the color mode chosen. CMYK has four separate channels plus a composite. RGB mode has three plus the composite. Each channel holds information about a particular color element in the image. Think of individual channels as something like the plates in the printing process, with a separate plate supplying each layer of color. You can often create interesting textures or special effects by applying filters to just one channel. Figure 5.16 shows the Channels palette (twice) with RGB and CMYK channels.
Figure 5.16 You can set preferences to show channels in grayscale or in their colors.
There are also alpha channels, which have several uses. They are used to define the placement of spot colors (Pantone, Focoltone, and so on). They also contain the maps for masks you create and want to save with the image to which you have applied them.