Crewel Work is a form of surface embroidery which is usually stitched using crewel wool on a heavier linen fabric (traditionally twill).
Crewel wool is finer than tapestry wool, it is a fine 2 ply. There is a wide variety of stitches that are used which makes it a very interesting form of embroidery.
It is a technique that dates back at least a thousand years. One of the earliest examples of this technique is on the Bayeux Tapestry.