Muscles are classified into three groups; skeletal (or striated), smooth and cardiac.
The main function of a muscle is to generate a contraction.
Skeletal muscle is voluntary muscle so requires a conscious activation or relaxation. Skeletal muscle is attached from bone to bone via tendons and has an action on the respective joints that it crosses. Skeletal muscle is also striated-which mean that it has alternating bands that are both light and darker in color producing a visually striated affect. The muscle has different color in these striations depending on the amount of myoglobin in the muscle. Myoglobin is stored oxygen, until it is required. Skeletal muscle is further broken down into slow and fast twitch fibers.
Smooth muscle is involuntary which means that you do not consciously activate the action of the muscle and is found in internal structures of the body in the walls of the tissue. Areas where Smooth muscle is found are blood vessels, stomach, bladder and intestines. The muscle is called Smooth as it is not striated.
Cardiac Muscle forms the majority of the muscle of the wall of the heart. It is involuntary like smooth muscle and striated like the skeletal muscle.