One of the biggest draws for Dragon Quest VII, other than its lengthy story, is the job class system in the game. Once you clear the Alltrades Abbey scenario, you will be able to freely change your characters into one of many job classes. Only so many are available from the start, but as you level them up to max, you will unlock more. All of the job classes in the game level up by having the characters participate in battles and after you win so many, the job class will level up.
To make things even more robust, in addition to the plethora of human job classes, you also have the ability to change into monster job classes. However, they work a little differently than human classes, as you need to find Hearts in order to transform into them. As you level up all of the job classes, you will learn new spells and abilities. Some of these will overlap, of course, and some spells/abilities can be kept whenever you change to a new class. However, some of the human classes will have their skills removed when changing classes, so be aware of that.