Fruit is a term generally applied to the products of flowers/fruiting bodies and often include seeds, where as a vegetable refers to a vegetative, non-reproductive part.
Using the correct botanical term is just a matter of good taste
The tomato, a fruit anyway you cut it, was legally declared a vegetable by the Supreme Court in 1893. This was done because there was a high Tariff (tax) on vegetables but none on fruit. One shipper tried to convince the courts that, as it was a fruit, they should be reimbursed past taxes paid for shipping. The Court declared that, since the tomato was used as a vegetable, it would be taxed as a vegetable.
The main difference being how tomatoes were used -- as a 'savory' like potatoes, eggplant, cucumbers rather than on its own as a fruit. Basically the Court said the tomato was a vegetable because most people thought it was.