Epithelial tissue forms the protective covering of the body and lines internal organs and cavities. It is involved in functions like protection, absorption, secretion, and filtration. There are three main types:

1. Squamous Epithelium: Made of thin, flat cells, allowing easy diffusion. Example: Lining of blood vessels and alveoli of lungs.

2. Cuboidal Epithelium: Composed of cube-shaped cells that aid in secretion and absorption. Example: Lining of kidney tubules and ducts of glands.

3. Columnar Epithelium: Consists of tall, column-like cells that help in absorption and secretion. Example: Lining of the intestine and stomach.