Non-sealed beams have a separate headlight assembly that takes a little bulb in the back of the reflector. Sealed beams are one big bulb; reflector, lens and all. I've heard that the non-sealed beams are more prone to failure due to vibration; don't know if it's true. I installed a 7" sealed-beam Wagner, and the difference is astounding; like I'd been riding around with a flashlight taped to my handlebars. I work a 3rd shift, so light's real important to me.
As Robb said, it looks way better than the goofy stock lamp, too.