By covering or lining an organ or other tissue, one side of the tissue is attached to the underlying substance of the body while the outer surface is free or unattached. The shape of mature cells is always seen next to the Free Surface. The attached surface is connected to the tissues so that the epithelium forms a continuous barrier or exchange surface. Epithelium does NOT have blood vessels in it. Therefore, its thickness is limited by the diffusion of nutrients from blood vessels to epithelial cells.
Epithelium is always very thin indeed.
Sometimes, modifications of surface cells require additional classification. For example, some tissues have cilia at the free surface. These are designated as ciliated columnar epithelium . You need to know how to identify each of the tissue types and to know examples of where such tissues are found or used in the body.