It is a big and long type of banana. An underripe banana has lower levels of sugar, making it better for diabetics. An average person however, digests ripened bananas better. Overripe bananas are chock full of antioxidants and sugars, very suitable for baking and making smoothies.
Fun fact: During shipping, unripe bananas are stored in cartons with the stem down first to reduce bruising on its skin. Also, its carton boxes have holes to encourage an uninterrupted stream of air through the bananas, preventing the bananas from ripening too quickly during transport.