According to About.com, cardiac muscles are found in the heart. Wikipedia explains that cardiac muscle is involuntary, meaning not under conscious control. It appears striated and is found in the walls of the heart, specifically the myocardium. Cardiac muscle cells rely on an available blood and electrical supply to deliver nutrients and oxygen, and remove waste products such as carbon dioxide.
University College London states that heart cells are self-contracting and continuously active throughout the life of an organism. Most cardiac muscle cells are y-shaped, shorter and wider than skeletal muscle cells, and mononucleated, meaning they only carry one nucleus per cell. Some of the cells in the heart automatically contract rhythmically, even without nerve impulses from the brain. These cells are often called “pacemaker cells.” Special disks located between the muscle cells help keep the entire heart pumping regularly and evenly.
Besides cardiac muscles, the human body relies on two other types of muscles. Smooth muscles are comprised of spindle-shaped cells, which line the hollow internal organs (besides the heart), and are generally not under conscious control. By contrast, MedlinePlus explains that skeletal muscles have a striated appearance and attach to the skeleton. These muscles are controlled consciously and permit movement.