A tissue consisting of sheets or bundles of cells (muscle fibres) that are capable of contraction, so producing movement or tension in the body. There are three types of muscle. Skeletal muscle produces voluntary movement (e.g. at joints); smooth muscle mainly effects the movements of hollow organs (e.g. intestine and bladder); and cardiac muscle occurs only in the heart.

http://www.innerbody.com/image/musfov.html Interactive tutorial identifying the principal muscles of the human body