The word "canvas" has come to mean any type of durable, simply woven fabric made from natural fibers. In ancient times, canvas was cloth woven from hemp; the root of the name comes from the same place as the word "cannabis." Modern canvas is usually made from linen or cotton. It is simply woven, as opposed to twill woven like denim.
Simple weave means that each string goes under, then over the next perpendicular string; it is the simplest and tightest possible weave. The simple weave not only means that canvas is cheap and easy to make, but that it dries quickly, which is important for a fabric that is often used and stored outdoors. It is also easy to oil (to make it temporally waterproof), and easy to remove the oil when it is no longer necessary.
Cotton and Linen
Linen is made from fibers of the flax plant, and maintains the natural oils of the flax plant after years of washing or years of being covered with paint. Linen is harder to work with than cotton--harder to size for clothing, and harder to stretch over a frame--but the finished product lasts longer. Cotton tends to dry out over the years and become either stiff of threadbare, depending on its environment. Linen can also be worked up into a wider variety of thread than cotton can. This means that there is a wider selection of linen canvases--in terms of smoothness and roughness--than cotton canvases. The great advantage of cotton over linen is that cotton is significantly cheaper.
Duck is the heaviest grade of cotton canvas. The name comes from the Dutch word for canvas: doek. It is typically made with twice as much fiber going in one direction as in the other direction. It is graded by numbers that run from 1 to 12; the higher numbers are for the lighter (ounces per square yard) grades. The grades are determined by the number of ounces in a 36-by-22-inch patch. Grade 1 (18 ounce) is the heaviest grade, and is used for cots and hammocks. Grade 5 (14 ounce) is used for heavy work clothes, and Grade 12 (7 ounce) is used for light clothing like windbreakers.
Before painting on canvas became popular, artists painted on wood. The earliest examples of canvas paintings are found in the 14th century, but it really became popular in the 15th century. The canvas is usually stretched over a wooden frame and prepared with a coating of glesso, a mixture of chalk and gypsum. The coating is used to prevent the paint from getting to the canvas, which decreases the life of the canvas. Some modern painters--notably Jackson Pollock--have experimented with painting directly on the canvas with no gesso.