Concrete slabsMany tile installations are laid directly onto concrete slabs by adding a layer of thinset and applying the tile directly. However the potential problem here lies in the two items, the concrete slab and the tile. Concrete, like everything, expands and contracts and it does so at a different rate than the tile that is adhered to it. Naturally when two attached items move at different rates, something has to give and in this case it's usually the tile. Wooden sub floors will also move at different rates. The resulting movement to the tile is called "tenting" and much as the name implies the tile looks like a little tent when forced together.
Cement backer board
Illustration of Ditra installation