These two cuts are best see here. From one side of the block is chutoro and the fatty end is otoro.
On the one side the fatty tuna and on the side is the leaner meat, still fatty but the density of the fat is very different. The belly nearer the skin has more collagen and the fat is the barrier the tuna develops as it never stops moving.
http://www.jamessuckling.com/mesubims-blog-tuna-tutorial.html
