Freestanding decks and decks built in areas that never experience frost can use floating footings that rest on top of the ground to support the deck. The sole purpose of these footings is to spread the load transfered from each support post to a larger area of soil to prevent the deck from sinking. We have seen some decks built with concrete cinder blocks as foundations. This is typically regarded as a lower quality building practice.
Freestanding decks that are not attached to a house or other stationary building may use floating footings. The reasoning is that the deck will be free to move along with the ground. If the deck is attached to a house you will be required to install footings below the frost line to prevent the deck from separating from the house when the ground begins to move. Large freestanding decks and freestanding decks built above 30" should use frost footings to prevent the deck from stresses caused as the soil moves unevenly.