When something is fruitful, it's productive — but you don't have to produce fruit in order to be fruitful. You could have a fruitful trip to the mall if you come home with a slew of snappy new outfits.
There's a famous line from the Bible: "Be fruitful and multiply." That gives you a pretty good sense of the word: fruitful activity multiplies or adds to what's already there, producing more of something. A couple is fruitful if they have children: the more children, the more fruitful. An artist is fruitful if they create a lot of artwork. A business is fruitful if it creates profits and expands. Anything fruitful is productive: it grows and produces, much like a fruit tree.