The difference between wall and floor tiles is actually quite simple. You can use floor tiles on the wall, but not on the floor. Wall tiles are thinner and less strong than floor tiles. This has to do with the resources used for both tiles. Furthermore, for floor tiles wear value important because it is (a lot) of walking.

Less intensive use
Wall tiles do not have to have such a high wear value as floor tiles, because you do not walk on wall tiles. These tiles are therefore used a lot less intensively and no sand or the like will rub over them.