There are a total of 7 chapters, bringing different surrounds,(deserts, rain forests, ice lands, etc), and have different missions in each chapter. You gain chapters through leveling up through doing missions for experience. Certain levels unlock chapters for you.
Your character gets stronger as you progress in the game, and do missions. You can obtain many items or bonus experience through chests from defeating enemies in missions. You gain points to increase your skill attacks powers when you master missions. When mastering a mission, a harder version will take its place, giving you more experience, for the price of harder goals. Example: You have to defeat 20 enemies. When you master the mission and it gets harder: You have to defeat 40 enemies.