Whether it's an original painting, or a digital photo, canvas needs to be stretched.
The canvas is wrapped over the sides of specially shaped wood and stapled onto the back. If the artist hasn't left enough spare canvas around the outside of the picture, it might need to be stapled on the sides of the wooden frame to avoid losing some of the picture. A good quality oil or acrylic painting generally does not need to be covered by glass.
Other fabrics can be mounted this way too - many fabrics look great hung on the wall. And without glass or a frame, they are light to hang.
Many canvas paintings are hung on the wall without an additional frame. However a standard frame can then be fitted around this if required.
An alternative style of frame designed for stretched canvas, is called the Float Frame.