Rewards are items your Players can redeem using the points they have accumulated via completing Challenges in the entire Project. Depending on the number of credits the Players have, they can redeem any Reward available in the store.
Creating Rewards is a great way to motivate your Players to complete more Challenges and to earn more points to redeem the goodies they want.
Achievements are badges that Players unlock by completing a particular action defined by the Project Administrators. The action is not limited to completing Challenges or accumulating points. Unlike redeeming Rewards where Players can pick and choose from a "shopping list", Players need to fulfill certain criteria to earn these badges.