CMU stands for concrete masonry unit and they are also referred to as concrete or cement blocks that are used in constructing houses and buildings. Concrete masonry units are pre-cast from concrete and other materials like sand, gravel, and various aggregates from cinder or clinker. The center part of these concrete blocks is made hollow to keep them light and for insulation purposes. In the US, concrete masonry units are more commonly referred to as cinder blocks because of the added cinder aggregate. In other countries, they are referred to as breeze blocks because of the breeze or ash content. Some people also refer to concrete masonry units as hollow blocks or simply cement blocks.

The size of CMUs may vary from one construction site to another. The shape of these concrete blocks though is typically rectangular with two or three hollow spaces in between. These hollow parts of the concrete blocks basically make them very easy to stack alternately with steel reinforcements typically used in between the cores. CMUs or concrete blocks are commonly used in constructing walls for buildings and houses. When walls are designed to bear minimal amounts of load, they may be created with lighter concrete materials like the CMUs. The CMU structure also makes them very easy to install using the classing stacking pattern in masonry. At least two hollow parts or cores are necessary to make these concrete blocks stronger during installation. In some instances, many construction units also create CMUs with three or four cores. There are even those who create specialty shapes like U-shaped hollow blocks to be used in specific parts of a building.

The components in creating concrete masonry units may also vary but most are composed with cement, sand, and gravel. Other aggregates may be added to the mixture in order to improve its strength. For walls that are designed to carry minimal load, standard CMUs created with cement, sand, and/or gravel may be used. For walls that are to carry heavier loads, these concrete blocks may be reinforced with more cement and less sand and gravel.