A fruit refers to a particular kind of fruit. Apples are a commonly eaten fruit.
Fruits refers to a number of different kinds of fruit. Oranges and grapefruits are fruits commonly used for making juice.
Fruit refers to either a particular kind of fruit or various kinds of fruit. "It is good to eat fruit." You are referring to fruit in general. "I have been eating fruit every lunch." I may have been eating just apples or a variety of different fruits.