Digital Mockups

In Web and Interaction design, digital mockups are now really easy to make and definitely the best way to test the interfaces in real conditions.
It can be simple wireframe mockups, made on photoshop or more dedicated software like Balsamiq that you will show as pdf slides to your users and ask for comments on what they see, their feelings and what they expect can be done.
It's also really easy now to create simple interactive mockups that will enable your tester to navigate as they would on a real webpage or interactive screen. The mockups can be made of wireframes, or more advanced with some visual design. You can use a software like Pixate , where you bring images to create the interface, then apply the interactions and can test on multiple devices (Well Pixate is unfortunately dead now... check this list of uxtools for more choices).
Some websites like Usertesting offer you a "user testing service". They send to many users your digital mockups with the tasks you want them to conduct and some questions. While they are doing the tasks, users' screens are recorded and they are asked to Think aloud. At the end, they answer some feedback questions like "What would you change if you had a magic wand?", "What was the most frustrating?"...
Mockup of a mobile app made on Pixate - see full project here