An organ is defined as a group of tissues that work together to perform a specific function. Examples of an organ include the stomach, brain and liver.
The tissues that make up organs are made up of groups of cells that work together. Cells are designed to do a specific job, like transporting messages or materials.
When organs work collectively together to perform a specific function, it is known as an organ system. This occurs in more advanced animals, such as humans and large mammals. An example is the digestive system, which is responsible for processing food into energy for the organism.