Boulders are used for rock gardens, large rock walls, retaining walls, sculptural placement rocks, water gardens, and more! In addition to being functional they are aesthetically beautiful, maintenance free, and last forever as is. The definition of a boulder is a large, smooth rock. When you build a retaining wall and use a large rock in the wall, this rock is an example of a boulder. A rock that has been detached from the earth by erosion or for other reasons and that is free to be moved about is an example of a boulder.