A canvas is a strong, coarse unbleached cloth made from hemp, flax, cotton, or a similar yarn, used to make items such as sails and tents and as a surface for oil painting. Traditionally canvases are stretched over wooden bars and stapled, pinned or nailed to the back of the wooden bars, this causes the canvas to be pulled tightly over the bars.
Oil, or acrylic paintings, are normally produced on canvas, and today you also see photographs, serigraphs, and giclee prints on canvas.
Some paintings are stretched on deep (thicker) bars and the painting continues around the sides of the piece. You will commonly see photographs or artwork printed on canvas and stretched on bars with the image continued and stretched onto the sides of the bars so as to not have stark white canvas showing.