What's the difference between an organ and a muscle?

Yes, each muscle is an organ. Just like your skin is an organ.

.

an organ (Latin: organum, "instrument, tool", from Greek όργανον - organon, "organ, instrument, tool") is a tissue that performs a specific function or group of functions. Usually there is a main tissue and sporadic tissues. The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are the nerves, blood, connective etc.

(http://en.wikipedia.org/wiki/Organ_(anatomy))

Muscle (from Latin musculus, diminutive of mus "mouse"[1]) is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to produce force and cause motion. Muscles can cause either locomotion of the organism itself or movement of internal organs.

(http://en.wikipedia.org/wiki/Muscle)

Well yeah sometimes organs are muscles eg. ur heart is an organ, which is essentially made of up cardiac muscle fibers. The muscle contracts to pump blood around the body. But not every organ is a muscle eg. skin.

Muscle is a tissue. It is composed of muscle cells. An organ is composed of four different types of tissues.