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 to 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, including in the arms, legs, and organs. Major veins, such as the superior vena cava and inferior vena cava, collect blood from different parts of the body and return it to the heart. Proper functioning of veins is crucial for overall health and circulation.