Joy is an emotion in response to a pleasant observation or a remembrance thereof. The reason for a joyful reaction is usually that some expectation or need has been satisfied. Joy is usually expressed as a smile, a laughter or exclamation of joy.
Joy differs from happiness in that it is an emotion. Happiness, on the other hand, is what we might think of as a feeling, which is more fleeting. Joy may be thought of as "the emotional dimension of the good life, of a life that is both going well and is being lived well."