Stretched canvas can be pre-primed with acrylic gesso or oil-based primer, making the surface ready to accept paint. Use acrylic gesso with acrylic paint and oil-based primer with oil paint. Without a primer, paint would soak into the weave of the canvas.
Canvas is generally made from either linen or cotton, natural fibers that are woven together and then stretched over a frame or glued to a panel. Each fiber comes in different textures or weights.
The texture depends on the weave, so a finely woven canvas is smooth and best suited to small, detailed works. A rough texture is preferred by painters who like bold brushstrokes and larger paintings.
The weight is dependent on the thread density and is measured in ounces per yard. The higher the weight, the better the quality. Try to avoid canvases with weights lower than 8-10 oz.