Gesso is one of the handiest tools in my layering arsenal, because it can do so many things. This video will give you a rundown of the basics: what it is, what it's for, and a couple of ways to use it.
Stencil with it. Above, the flowers on the left side were applied by stenciling with gesso over a bare book page, and then glazing over the whole thing. This exploits the difference in the way the gessoed and ungessoed area grabs onto the glaze. The gessoed areas are also slightly dimensional. I did the same thing with a brocade stencil below, and some really old, thick gesso. The effect is similar to that of texture paste.
Paint with it. Using gesso instead of paint will give you blindingly white whites, and deep, dark blacks. All the circles on the page above were done with black and white gesso. I often use black and white gesso when I'm painting eyes, to give a very dark pupil, and a bright highlight.
Make magazine images accept paint. The face above started life as a magazine ad. I often apply magazine images, and then gesso over the top of them, leaving just the faintest amount of the original image showing through. That gives me a nice facial structure on which to paint my own faces. The black blacks in this pieces were done with black gesso, and the face was painted out with white gesso.