Bobbin work is putting a decorative thread in your bobbin and a clear thread on top and sewing on the reverse side of your fabric. The bobbin thread which is usually much thicker will then become the decorative element on your fabric. Couching is taking a heavy thread or ribbon and laying it on top of your fabric and then using a small zig zag stitch with clear thread or complementing thread and stitch the heavy thread in place. I hope that helps!