Canvas is usually stiffened cotton. The holes are larger than in other evenweaves, which means canvas can accomodate some of the thicker embroidery threads. The stiffness of the canvas also means it is suitable for heaviery coverage than other evenweaves. It is usually mounted on stretcher bars rather than in a hoop or on a scroll frame, although the finer canvases (i.e. 24-count) can be worked quite handily on a scroll frame or Q-Snaps™.
Counted canvas is the name given embroidery on canvas ground fabric that does not fall into the traditional definition of needlepoint. It usually involves unusual stitches and threads, doesn't always completely cover the canvas, and is often geometric in design rather than pictorial.
Most of my more current classes are on canvas. Designers often work in a series of designs, exploring certain aspects of stitching. This often involved working on a particular fabric for a period of time.
Sometimes stitching information is imparted by talking rather than stitching (I know — hard to believe). Such classes are often well-suited to a guild chapter program or as a precursor to a stitching class.