The general characteristic of epithelial tissue is that the cells are closely packed together. Since they form the linings of organs and body cavities, they have at least one side that's not in contact with other cells. They lack blood vessels and obtain nourishment through diffusion.
Because they are so closely packed, epithelial cells have little matrix between them. They're separated from underlying cells by a basement membrane.
Epithelial tissue is responsible for protecting the body, secretion and excretion, absorption and allowing the organism to sense the outside world. There are three basic types of epithelial cells: squamous, cuboidal and columnar. Epithelial cells can come in sheets of one or more layers. The squamous cells are thin and look like scales with irregular edges. They make up the cells that cover the surface of the body as well as lining the mouth and the esophagus.
Cuboidal cells resemble tiny cubes and are nearly as tall as they are wide. These cells line some of the body cavities and the glands. Columnar cells are much taller than they are wide and are found in the stomach and the intestines. Another type of columnar epithelial cell is found in the respiratory system.