Canvas is a type of extraordinarily thick and stiff fabric used mainly in making sails, tents, marquees, backpacks, etc. The fabric is also used as a painting base. Artists cut the fabric into pieces as par the size of the paintings. Canvas is stretched over the stretcher (wooden frame). Occasionally it is coated with gesso before painting is done. Gesso is a brittle kind of material that starts cracking after getting dry. Other types of primers are available in the market that gives a naturally elastic base to the picture. Those primers are light. That is why the canvas doesn't become heavy after application.
For past few years it has emerged as a style to use unprimed canvas or "raw canvas" as the base of oil paintings. Modern artists like, Francis Bacon, Kenneth Noland, Helen Frankenthaler, Dan Christensen, Larry Zox, Jackson Pollock, Ronnie Landfield, and some lyrical abstractionists and others have tried this bare material and are quite satisfied with the effect and texture of the paintings. Unprimed canvas is called bare canvas.
Earlier canvas used to be made of linen. It was a stiff brownish sturdy fabric . Linen can be used for oil paintings. Cotton canvas started being used as the painting base sometime in the early twentieth century. It is also called the " Cotton duck ". This one has a flexible stretch and the weave is very brush friendly.
Prepared canvas is the most popular base for painting nowadays. This is quite user-friendly for both the beginners and the artists. It is sometimes adhesive to the cardboard back. The problem is that these are only available in some particular sizes. That is why it is a problem for the artists willing to paint on a different kind of size and shape such as, triangle or a slender rectangle. The primers on these materials are not acid-free. So the painting gets dark patches .
While choosing the framed canvas art one must take care about the oils, paints and the base has been used for the painting. Otherwise discoloration will make you feel like sheer wastage of money.