Ultimately a prototype is just a tool for learning and discovery. So what you build depends on what you're trying to learn or work out. I've built prototypes as

Ultimately a prototype is just a tool for learning and discovery. So what you build depends on what you're trying to learn or work out. I've built prototypes as powerpoint decks, basic HTML/CSS, and as apps built in Rails or PHP. Sometimes with a database backend - or with some spoof of one.