There are four types of Prototypes:
1) Proof of Principle
2) Form Study
3) Visual Prototype
4) Functional Prototype
Your design may go through all four or only one type before being produced. Each type evaluates, analyses and reapplies this information to the original design, improving it at each stage. At each stage the prototype is tested multiple times as the design is improved into a final product.
Proof of Principle is a functional model meant to test a portion of the design to determine if it will work as anticipated. Tests in this phase are range of motion, sensors, product architecture and mechanics.
Form Study is used for the visual appearance and usability of a product.
Visual Prototype looks at everything from the color scheme, texture and other design elements.
Functional Prototype is the final, fully functional model of the end product. This is to make sure that the product will function as anticipated.