standard trackballs have a ball that you manipulate with your fingers, and look something like this
the thumb trackballs look like this
now, i don't have extensive history with the thumb models, but i have tried a couple of them out and did not find them to be as comfortable, nor could i point as accurately. i do have two finger operated trackballs, and old school logitech model with a smallish ball centered between two buttons. nice, but too simple. i also have a logitech marbleman fx which has a large ball and more buttons. the resting position of the hand is more natural too (with your hand in almost a shaking hands position). i use this at work interchangably with my very comfortable logitech mouseman wheel.
the only thing i like better than either of those last options is my macbook's trackpad.