Whip stitch is a simple diagonal stitch with a whipping/wrapping motion.  It is often used to sew two pieces of fabric together, and it can be used in appliqué to attach a small piece of fabric to the ground fabric (in this regard it is similar to stab stitch, but the stitches are slightly longer and diagonal). It can also be used as a surface stitch, and is a very simple, effective way to create a rope effect in bead embroidery. You can vary the length of the stitches, or alternate between different beads, or combinations of beads and sequins in order to achieve various interesting effects.

The image shows whip stitch used to apply a piece of fabric, as a beading stitch and as a surface stitch.

This stitch should not be confused with the action of whipping another stitch: whipping is done by diagonally wrapping a thread around an existing line of stitches, the needle is always inserted from the same side so that the slanted wrappings are parallel and crucially it does not pierce the fabric.  See the entry for Whipped as a structure.