Epithelia are layers of cells that cover surfaces and line cavities in the body. They serve various functions, including protection, absorption, secretion, and sensation. Epithelial tissues are classified based on the number of cell layers and the shape of the cells, such as squamous, cuboidal, and columnar. These tissues are anchored to underlying structures by a thin layer called the basement membrane. Epithelia are avascular, meaning they do not contain blood vessels, and receive nutrients through diffusion from nearby tissues. They play a crucial role in maintaining the body's integrity and facilitating interactions with the environment.