When some one is engaged with a gamification experience, one of the key elements is often some type of reward structure. In most obvious situations, the rewards are the in guise of points or badges or a similar element. The reward structure, however, plays as much into the continual motivation of the learner as does the form of the reward. I have written about this before in Measurement vs. Completion Achievements in Gamification.
However, there are many different ways to look at rewards and achievements. Understanding the concepts from several different angles helps with understanding the best methodology of applying rewards to help motivate learners. One way to think of them was developed by Game Designers Neal and Jane Hallford and written about in the classic book Rules of Play by Salen and Zimmerman.