The plant likely originated in India, and has been part of the human diet since the aceramic (non-pottery producing) Neolithic times, being one of the first crops domesticated in the Near East. With approximately 26% of their calories from protein, lentils and generally any pulses or legumes have the third-highest level of protein, by weight, of any plant-based food after soybeans and hemp and is an important part of the diet in many parts of the world, especially in the Indian subcontinent which has large vegetarian populations.
A variety of pulses or lentils exists with colors that range from yellow to red-orange to green, brown and black. Red, white and yellow lentils in some cases are peeled, i.e., are those that have their skins removed before being sold. There are large and small varieties of many lentils (e.g., Masoor Lentils, shown in photos here). Lentils are sold in many forms, with or without the skins, whole or split.
Culturally, other pulses are sometimes called lentils but are actually beans or peas, e.g. “black lentils” (urad beans).