The DS Lite Guitar Hero Stylus is exactly the same idea as the Jammin' Guitar Pick stylus that Naki released to go with Jam Sessions. It's a stylus attached to a guitar pick. You hold the pick and use the stylus end on the screen, which sounds, at least, a little more guitar-like than just playing the DS with a little stick.
The major differences between the two products? The Guitar Hero set has the Guitar Hero logo on the picks, and doesn't come with as many.