Joy is the emotion of pleasure and happiness. Joy can also be the very thing that delights you. You might find great joy in learning new vocabulary. Or not.
Joy can also be used as a verb. You can joy in the fact that you have no homework for the weekend. Or, you might want to rejoice when the teacher announces that the test has been canceled. It's a real joy to use the word as an adjective, too. For example, receiving straight A's probably makes you extremely joyful.