1. the emotion of great delight or happiness caused by something exceptionally good or satisfying; keen pleasure; elation: She felt the joy of seeing her son's success.
2. a source or cause of keen pleasure or delight; something or someone greatly valued or appreciated: Her prose style is a pure joy.
3. the expression or display of glad feeling; festive gaiety.
4. a state of happiness or felicity.
1. the quality or state of being happy.
2. good fortune; pleasure; contentment; joy.
Looking at the definitions, it doesn't seem like there is much of a difference at first, does there. Read closer - at the end of the day, joy isn't always something that comes from without, it comes from within. Also, it is more complex than simple happiness. It something we do more than something that is. It is called a noun, but it is really a verb, adjective, or adverb really.
Not mentioned in the dictionary, but something that I feel and have been taught in church is that joy is a choice, at the end of the day.
Not as easy as it seems. But joy is promised by being is His word and "hearing" His words. It is promised.
I have to believe this.