Epithelial tissue is one of the four main types of tissue in the body, alongside connective, muscle, and nervous tissues. It serves as a protective layer covering the surfaces of organs, both inside and outside the body. Epithelial cells are tightly packed together, forming sheets that can be classified based on their shape—squamous, cuboidal, or columnar—and the number of layers—simple or stratified. This type of tissue plays several essential roles, including absorption, secretion, and sensation. For example, the epithelial lining of the intestines helps absorb nutrients, while the skin acts as a barrier against environmental hazards. Epithelial tissue is also involved in the formation of glands, such as sweat glands and salivary glands, which produce various substances.