During my first trip to Iceland, I was frustrated because I didn't own any Neutral Density Filters. A Neutral Density filter is a fancy name for "dark grey glass", whose sole purpose is to let in less light. If you let in less light, you can then use longer exposures, which can allow you to take dream-like wispier waterfalls or clouds. The densest of neutral density filters can also be used to empty a building via a 4-hour exposure. In situations like these, anything that stays still during those 4 hours will be rendered in the image, but anything that moves will be "averaged away" and become invisible. Architectural photographers use this technique a lot.
