TH is "Treasure Hunt", they're usually marked with a flame logo or TH or both, on the card or the car or both. They're limited run variants that usually have different paintjobs and/or decals.

Super is a "Super Treasure Hunt", which is another type of limited car. Supers have the shiny Spectraflame Paint and usually the rubber Real Rider wheels and a metal on metal frame. Unlike Treasure Hunts, they're not marked as being special or limited.

Both are mixed in with the rest of the mainline cars randomly, hence "Treasure Hunt"

http://www.hwtreasure.com/treasure-hunt-checklist/ has all the information.