Epithelium is a type of tissue that forms a protective layer on surfaces throughout the body. It lines organs, cavities, and structures, serving as a barrier and playing a role in absorption, secretion, and sensation. There are different types of epithelial tissue, including simple epithelium, which has a single layer of cells, and stratified epithelium, which consists of multiple layers. Epithelial cells are tightly packed together, minimizing the space between them. This arrangement helps to protect underlying tissues from damage and pathogens. Additionally, epithelial tissue is involved in processes like gas exchange in the lungs and nutrient absorption in the intestines.