A vein is a type of blood vessel that carries deoxygenated blood back to the heart. Unlike arteries, which transport oxygen-rich blood away from the heart, veins have thinner walls and often contain valves that prevent the backflow of blood. They are an essential part of the circulatory system, helping to maintain proper blood flow throughout the body. Veins are found throughout the body and are categorized into superficial veins, which are close to the skin's surface, and deep veins, located deeper within the tissues. Major veins include the superior vena cava and inferior vena cava, which return blood from the upper and lower parts of the body, respectively.