From The School of Biomedical Sciences Wiki
An organ is a combination of specialised tissues formed from differentiated cells which work in unison to perform a specific function within the body. Examples include the lungs, heart, liver and eyes. Organisms rely on healthy organs for every day functions of the body such as maintaining homeostasis and respiration.
An organ is a collection of tissues that work together to achieve a mutual purpose. Examples of organs include the heart and the brain, they have very specific and important functions in many living things. The heart, for example, is a collection of tissues: myocardial, elastic and endothelial. These work together acting as a pump to allow blood to circulate the entire body of an animal.