The heart is an organ which is made from muscle.
Organs are a collection of tissues that are specialised for their purpose. Muscle is a type of tissue. The heart is made up of a collection of different tissues such as nervous tissue, connective tissue and cardiac muscle (muscle specialised for the heart) and blood (which is classified as a tissue as it is a collection of cells!).
However, people may refer to the heart as a muscle as it is mainly muscle (which is a good thing as it needs to pump masses of blood around the entire body).
Hope this helps; let me know if I can do anything else:)