Kneehighs come in two distinct styles, kneehighs and over the knee socks. Regular kneehighs will only go up to (at most) the bottom of the kneecap. In absolute terms they are largely just very long socks.
Over the knee socks differ in that they are designed to be pulled over the knee and rest above the kneecap. At first look they can be mistaken for thighhighs; they are much shorter than thighhighs and can't be pulled up much more than a few inches above the top of the kneecap.