According to botanists (those who study plants) a fruit is the part of the plant that develops from a flower. It's also the section of the plant that contains the seeds. The other parts of plants are considered vegetables. These include the stems, leaves and roots and even the flower bud.
The following are technically fruits: avocado, beans, peapods, corn kernels, cucumbers, grains, nuts, olives peppers, pumpkin, squash, sunflower seeds and tomatoes. Vegetables include celery (stem), lettuce (leaves), cauliflower and broccoli (buds), and beets, carrots and potatoes (roots).
From a culinary standpoint, vegetables are less sweet or more savory and served as part of the main dish. Fruits are more sweet and tart and are most often served as a dessert or snack. Both fruits and vegetables can be made into juice for a refreshing beverage. Some fruits are "grains" or "nuts" or "seeds" and are served accordingly.