Mockup: A mockup is a high-fidelity render of your design that showcases how the finished product will look. Prototype: A prototype is an early model of a product that focuses on functionality and gives your stakeholders a taste of the final version.
Mock-ups make it possible to do usability testing early in the development process. Mock-ups incite and legalise experimentation as they are inexpensive to alter. Mockups focus on content and functionality and turn attention away from details of graphic design.
Design determines the look and feel of a website, while development determines how it functions, the interaction logique and the architecture of the project.