Epithelial tissues form linings and coverings. The cells are attached to each other, they have a free surface and an attached surface, and they sit on an acellular basement membrane, about which I will say more later. If the cells are flat, the epithelium is referred to as squamous . This is typical of epithelia through which gases need to move, such as the capillaries that bring blood to the tissues or the alveoli of the lungs, which take up oxygen and release carbon dioxide. These squamous epithelia are specialized for diffusion.
Some squamous epithelia have multiple layers, and are referred to as stratified .
The epidermis of human skin is a good example of this. These epithelia are called cuboidal , and are typical of many glands and the tubules of the human kidneys. These cuboidal epithelia are specialized for secretion and absorption. When tall cells form an epithelium, it is referred to as columnar . Columnar epithelia usually imply very active secretion and absorption.