It is considered to be a fruit (in the botanical sense, that is the correct designation) because the tomato comes from a flower. A vegetable on the other hand, comes from the vegetative part of the plant - such as spinach, lettuce, carrots, potatoes, beets, cabbage - we are eating roots, stems or leaves. However, in common use, many plants we grow are called vegetables, though they are technically fruits - cucumbers, melons, squash, peppers, eggplant - nothing much wrong with using the word vegetable in everyday conversation.