Internal motivation is the drive inside us that inspires us to reach our goals and do a great job. The reward is the good feeling we get for overcoming a challenge.
I recently read a good quote about happiness that is attributed to Deepak Chopra: "Be happy for no reason, like a child. If you are happy for a reason, you're in trouble, because that reason can be taken from you."
I think these words are very wise, and I think they are true for motivation, as well as happiness. Although I am not against an occasional reward for good results, I think we should strive to be motivated because we just want to do a good job. Remember how good it felt the last time you aced a big history test? Or the great way you felt once you finished a big science project? That good feeling comes from inside you, and nobody else can take that away. That's the sort of reward you should seek, because those little triumphs represent growth of character. They are steps toward becoming a better student and a happier person.