A vein is a blood vessel that carries blood toward the heart. There are four main types of veins: pulmonary and systemic, and superficial and deep.
About.com Biology states that each of the four types of veins is unique. Pulmonary veins are responsible for transporting oxygen-rich blood from the lungs to the left atrium of the heart. Systemic veins collect oxygen-poor blood from all parts of the body and return it to the right atrium of the heart. Superficial veins are close to the surface of the skin whereas deep veins are located inside muscle tissue. Deep veins are usually located near a corresponding artery with the same name, but superficial veins are not.
Veins can be as small as 1 millimeter or as large as 1.5 centimeters in diameter. About.com Biology explains that part of the body's system of veins is comprised of small venules that receive oxygen-poor blood from capillaries. The venules transport blood to systems of larger veins, which eventually lead to the superior vena cava or the inferior vena cava. The venae cavae lead to the right atrium of the heart, where the blood is returned to flow through the cardiac cycle.