A concrete masonry unit is a standard-size rectangular block used in building construction. CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them. They’re good for creating walls that provide protection against the elements, such as strong winds. They can also be used for projects such as garden walls and planters, retaining walls, foundations, steps, and firepits. Solid concrete blocks are usually heavier than hollow blocks, but they can be less expensive.