Skateboards are cool and all but their main downfall is that you can only use them on hard surfaces. You can ride them on pavement and in skate parks, but other than that, that is about it. You can not even ride over big rocks on the pavement. One of the worst things that can happen when riding is that you'll be just cruising and then out of nowhere hit a rock and fly off your board.
Off road skateboards are not limited to just one terrain. They can be used on all sorts of surfaces like dirt, grass, gravel, hard sand, and even pavement.
You can convert a normal skateboard into an off road skateboard by switching the wheels with larger rubber ones. This will allow the board to go over some grass and dirt surfaces, as well as those wheel biting rocks. This is an alright option, but if you want a board that can handle rougher terrains, you'll need something a little heavy duty.
Mountainboards are large off road skateboards with big all terrain tires and bindings. Some may not have bindings, but most will have them. The bindings may be snowboard style or just a simple single strap that lets you easily remove your foot. Brakes can also be found on a mountainboard to make it easier to stop.
A mountainboard can handle rolling over sticks, rocks, leaves, and bumpy terrain. The wheels easily roll over most terrain. Basically, you can use it where there is a slope.