Epithelium is a specialized layer of epithelial tissue that covers body surfaces, lines internal cavities, and forms glands. This tissue type is characterized by having closely packed cells that are attached to the underlying connective tissue by a basement membrane. The primary functions of epithelia include protection, filtration, absorption, and secretion. Depending on the underlying function and location within the organism, epithelia can be classified based on the shape of their cells—such as squamous (flat), cuboidal (cube-shaped), or columnar (tall)—and the number of cell layers present, forming single-layered (simple) or multi-layered (stratified) sheets.