ANATOMY

Epithelium : is comprised of cells which cover

•  the exterior surfaces of the body (skin, front of eye)
•  line  the internal closed cavities of the body (thoracic [lungs], pericardial [heart] and abdominal [guts]) and
• those body tubes which communicate with the exterior (the alimentary [GI tract], respiratory [lungs] and genitourinary tracts [kidneys, bladder etc]).

Epithelium also forms the secretory portion of glands and their ducts, and the receptors of certain sensory organs (e.g. taste buds; olfactory [smell]cells).

The cells forming the epithelium are in close contact with one another.

They may be arranged in

• multiple layers, as in the covering of the exterior surfaces of the body where protection and impermeability are primary requirements, or
• in a single layer, as in the lining of most of the internal surfaces of the body. 

POLARISATION:

All epithelia exhibit a free surface at their apex, and on the opposite surface, they adhere to a basement membrane. Thus they are polarized cells. This polarity is maintained by the various intercellular junctions, which bind the cells together and by molecules which bind the epithelial cells to the basement membrane. Thus the apical side of an epithelial cell is generally quite different (structurally and biochemically) from the basal side. 

Blood vessels do not normally penetrate the epithelium,Nutrition of the epithelium thus depends on the diffusion of metabolites through the basal lamina.

Since epithelia cover free surfaces of the body, they are often subject to abrasion, thus they are constantly replaced. As cells at the free surface are sloughed off or die, cells sitting on the basement membrane (basal cells) divide and differentiate into the various cells which comprise the epithelium.

Classification of Epithelia.

Classification of the various epithelia of the body is based solely on the arrangement and shape of the cells. The terminology used is thus unrelated to function and serves only for descriptive purposes.  
Simple: one cell thick: 

stratified, two or more cells thick.  

Epithelia (both simple and stratified) are further described according to the shape of the cells forming the surface layer.

The individual cells comprising the surface layer of the epithelium may be described as:

squamous, where the width and depth of the cell is greater than its height (like a fried egg);

cuboidal, where the width, depth and height are approximately the same (like a cube):

columnar, where the height of the cell appreciably exceeds the width or depth.  

To complicate this simple classification, two special categories of epithelium are typically included, namely pseudostratified and transitional epithelium.