Gesso is a material used to prepare canvas, linen or other surfaces prior to the application of paint. Once the canvas has been stretched, gesso is painted on, usually two layers thick, to provide a base for the paint. Gesso provides a bright white surface to paint on and prevents the deterioration of the canvas. (Oil paints will rot the canvas if applied directly to it.) It is pronounced like jello, but with s's.
There are two major types of gesso, traditional gesso and acrylic gesso. Traditional gesso is made from marble dust (powdered calcium carbonate), to make the surface white and rabbit skin glue, to adhere the marble dust to the canvas. Traditional gesso was the only way to prime a canvas until the mid 20th century, when acrylic paints and acrylic gesso were developed.
Acrylic gesso is made from an acrylic base and a white pigment. Acrylic gesso is generally similar to white acrylic paint. Acrylic gesso usually has a stronger adhering agent than normal acrylic paint. It also has a higher concentration of whitening pigment (usually titanium dioxide).
The first coat of gesso on a canvas is generally more watery, so that it will soak into the canvas better. The second coat can be thicker, to create a strong base for the paint and to create texture.
Traditional gesso is used by those artists painting oil paintings who wish to paint in a more traditional manner. It does make a base that is a brighter white than acrylic gesso. Traditional gesso is also much more work to prepare and apply than acrylic gesso.
Acrylic gesso can be used for canvases that will be painted on with acrylic or oil paints. It can also be applied to many other surfaces easily. It is easier to apply and work with than traditional gesso.