A group of tissues that work together to perform a specific function is called an organ. The human body has many organs, and each has its own specific function. In some cases, groups of organs also work together to serve a larger function, and this is known as an organ system.
Organs that are found within the body's main cavity are known as the visceral organs, or viscera. The digestive organs, for example, are all visceral organs. This includes the stomach, whose function is to churn and mix food; the small intestine, which is a group of tissues that work together to break down and absorb nutrients from food; and the large intestine, where water is reabsorbed into the blood stream. Each of these organs contains several types of tissue, but they all work together to accomplish the organ's major function. The organs of the digestive system all work together to complete the system's main function -- breaking down food, absorbing nutrients and excreting the end products.
The body also contains many organs that are found outside the visceral cavity. These organs are also comprised of groups of interworking tissues. Examples include the skin, which is the largest organ in the human body, and the eye, which is an organ made of more than 30 different tissue types.