Epithelial tissue is a continuous sheet of tightly packed cells, held together with very little intercellular material, that covers body surfaces, lines internal cavities/tubes, or forms glands. As a group it handles protection, absorption, filtration, excretion, secretion and sensory reception, with individual sub-types specialising in particular combinations of these roles.
Epithelia are classified first by layer count. Simple epithelium (a single layer) is suited to absorption, secretion and filtration, and comes in five forms: squamous (thin, flattened cells; kidney glomeruli, lung air sacs, vessel linings — diffusion/filtration), cuboidal (cube-shaped cells; kidney tubules, gland ducts, ovary surface — secretion/absorption), columnar (tall cells with basal nuclei; digestive tract lining — absorption/mucus secretion), ciliated (columnar cells with motile cilia; bronchioles, fallopian tubes, uterus, trachea — mucus propulsion), and pseudostratified (columnar cells of unequal height whose staggered nuclei make a truly single layer look multi-layered; trachea/upper respiratory tract). Cells specialised purely for secretion form glandular epithelium, divided into unicellular glands (e.g. goblet cells) and multicellular glands, and further into exocrine glands (release product via a duct — classified by structure as simple/compound, by secretory-unit shape as tubular/alveolar/tubulo-alveolar, and by secretion mode as merocrine/holocrine/apocrine) versus ductless endocrine glands (release hormones into surrounding fluid).
Compound (stratified) epithelium (more than one layer) trades away most secretory/absorptive capacity for resistance to chemical and mechanical stress — it is chiefly protective. Its four forms are stratified squamous (keratinised in dry skin epidermis; non-keratinised in the moist oesophagus/mouth/conjunctiva/vagina lining), stratified cuboidal (sweat and mammary gland ducts), stratified columnar (limited distribution — pharynx, male urethra, some glandular ducts), and transitional epithelium (ureters, urinary bladder, part of the urethra — allows stretching while staying protective).
Whatever its layer count, an epithelial sheet's cells are linked by three types of cell junction: tight junctions (seal cells, preventing leakage across the tissue), adhering junctions (cement cells together mechanically) and gap junctions (connect adjoining cells' cytoplasm for rapid ion/small-molecule communication).