Dish Network dishes, a Dish 300 (used to pick up satellite locations 61.5, 148, and use to be used to pick up 110 or 119)has a single LNBF (the thing in front of the Dish that looks back towards the dish.) A Dish500 has two (though most of the time it looks like one) LNBFs that look at 110/119 at the same time. Basicly, a SuperDish picks up signal from one more satellite location. A SuperDish comes in two varieties, one that picks up 105/110/119 and another that picks up 110/119/121, the LNBF that picks up 105 or 121 is different than the ones that pick up 110/119/61.5 and 148. (Note: 61.5, 105, 110, 119, 121, and 148 are different satellite locations in the sky, depending on your area you would point your dish in different directions, your receiver tells you what area you need to point your dish.)
Yes you will still need an OTA antenna to pick up OTA HD channels with a SuperDish, though Dish may or may not offer your local channels with the SuperDish, they are not HD, they are SD only.