A structure composed of two or more tissues that work together to perform a particular function is called an organ. The liver is the largest as well as the heaviest organ in the human body. Other major organs include the heart, brain, lungs, kidneys and stomach.
An organ system is formed when a group of organs work together to perform a specific process. For example, the digestive system is made up of the stomach, small intestine and large intestine. The endocrine system is comprised of the thyroid gland, the pituitary gland and the thymus. Several organ systems exist in the human body.