Artists use small (or sometimes much larger) pieces of canvas as a base for their works of art. This canvas is stretched across a wooden frame called a stretcher, and is coated with gesso before it is to be used (although some modern artists, such as Francis Bacon and Helen Frankenthaler sometimes paint onto the bare, unprimed canvas).
You can also buy small pre-prepared canvases which are glued to a cardboard backing in the factory, and precoated, but these are only available in certain sizes and they are not acid-free so their lifespan is extremely limited. They are usually used for quick studies. Pre-gessoed canvases on stretchers are also available. Professional artists who wish to work on canvas usually prepare their own canvas in the traditional manner.
One of the most outstanding differences between modern painting techniques and those of the Flemish and Dutch Masters is in the preparation of the canvas. "Modern" techniques take advantage of both the canvas texture as well as those of the paint itself. A novice artist often finds it nearly impossible to approach the realism of such classic art, despite excellent skill in applying the paint, and, in utter frustration, might wonder what he is "doing wrong" The answer is relatively simple. Renaissance masters took extreme measures to ensure that none of the texture of the canvas came through. This required a painstaking, months-long process of layering the raw canvas with (usually) lead-white paint, then polishing the surface, and then repeating. The final product had no resemblance to fabric, but instead had a glossy, enamel-like finish. This may seem an extreme measure for the modern painter, but is absolutely crucial if photographic realism is the end goal. This is just one of the not-so-secret "secrets" of the great Masters; there is no substitute for such devotion to detail.
With a properly prepared canvas, the painter will find that each subsequent layer of color glides on in a delightful "buttery" manner, and that with the proper consistency of application, (fat over lean technique) a painting entirely devoid of brushstrokes can be readily achieved. Without such a flawless surface, the painter will be forever consigned to something less than complete mastery of the work.