Wikipedia defines Fruit and Vegetables as follows:
"There are three definitions relating to fruits and vegetables:
Fruit (scientific): the ovary of a seed-bearing plant,
Fruit (culinary): any edible part of a plant with a sweet flavor,
Vegetable: any edible part of a plant with a savory flavor."
A fruit is actually the sweet, ripened ovary or ovaries of a seed-bearing plant. A vegetable, in contrast, is an herbaceous plant cultivated for an edible part (seeds, roots, stems, leaves, bulbs, tubers, or nonsweet fruits). So, to be really nitpicky, a fruit could be a vegetable, but a vegetable could not be a fruit.
The Nutriquest team offers a similar answer, adding that most fruits are sweet because they contain a simple sugar called fructose, while most vegetables are less sweet because they have much less fructose. The sweetness of fruit encourages animals to eat it and thereby spread the seeds. The site also presents an interesting list of fruits that are often thought to be vegetables:
tomatoes
cucumbers
squashes and zucchini
avocados
green, red, and yellow peppers
peapods
pumpkins
But hey, what about the nut? Well, according to our friends at The Straight Dope, a nut is actually a "a dry, one-seeded, usually oily fruit."
Potato potahto, tomato tomahto, let's call the whole thing off.
