1. Verifying mental models is where we show a prototype to a user or buyer and ask them if what we are showing brings any value for them. In such situations the moderator or presenter tells a story using the prototype as an artifact. The purpose here is to engage in a conversation to validate ideas and even generate ideas.
2. Usability testing with a final product or prototype is to verify if the product is usable. In such sessions the user is given the prototype and a script and asked to use the product. Moderator just observes and guides the user. The purpose here is to get detailed specific feedback on what usability issues need to be addressed.