Difference between 水果 and 果实
「果實」matches the botanical definition of fruit, and「水果」refers to the colloquial usage. The Wikipedia article sums it up nicely:


  In common language usage (水果), "fruit" normally means the fleshy seed-associated structures of a plant that are sweet or sour, and edible in the raw state, such as apples, bananas, grapes, lemons, oranges, and strawberries. On the other hand, in botanical usage (果實), "fruit" includes many structures that are not commonly called "fruits", such as bean pods, corn kernels, tomatoes, and wheat grains.