Stratified epithelium is classified by the cell type on the uppermost layer. Notice that is true for the pictures shown below. They are typically found in locations where constant abrasion is likely, such as skin. The epithelium at these locations have varying thickness of Kertain on top, and thus are classified as Keratinized. In other words, the uppermost layer of cells are dead.
Because the uppermost layer of cells are regularly sloughed off by abrasions, they are constantly replenished by an actively dividing basal layer. The basal layers tend to be more cuboidal in shape and the cells being pushed outward gradually take on the flattened morphology. In keratinized epithelium, the cells also produce keratin as they move upward, forming a layer of keratin as the cells die. This layer of keratin forms the barrier to protect against excessive water loss and foreign substances invasion.