Rapid prototyping is an amazingly effective way to get ideas in front of both stakeholders and users quickly and affordably for testing and validation. There are many ways to create prototypes. They may be as simple as a white-boarding session, paper cut outs used to simulate changing interfaces and content as a user talks through what they would do, to cliackable *.PDF or HTML files.
As a tool, prototypes can teach us a lot about our product and ideas with very little investment in time or money. what might be a simple hypothesis or subjective opinion can often be validated or in validated just by building it out, even in a very primitive form.
Above is an idea for a simple scheduling application. This was created in a tool called Balsamiq as a clickable *PDF that mimicked a site in a browser. It was created in a matter of hours, allowing me to quickly get it in front of users for invaluable qualitative user research.