People (especially kids) like getting into shouting matches about whether a tomato, say, is a fruit or a vegetable. Of course, the problem is that people are using different definitions. From a scientific standpoint, a fruit is the seed-bearing pod of a plant, and a vegetable is essentially anything plant-related. So from that standpoint, a tomato is certainly a fruit, and is vegetable as well (as are all fruits). From a culinary standpoint, though, we have this distinction among some edible plants, into fruits and vegetables (there are plant-derived foods not considered either, of course, such as cereals). And of course, these are culturally determined (I've heard in some cutures rice, for example, is considered a vegetable and in some a cereal). Not all fruits are biologically fruits, and some vegetables are. So cucumbers and tomatoes are vegetables, even though biologically they're fruits, while strawberries (technically bits of stems, not fruits) are fruits, and I'd be half-inclined to consider rhubarb a fruit too, even though it's just a stem.
An interesting rule of thumb I've heard which seems to work pretty well at least in my culture (figure standard Western, European, etc.):
Fruits are those edible plant-bits which are predominantly sweet or sour. The others are vegetables.
Tomatoes and cucumbers aren't sweet enough to be fruits. Nor are peppers, even carrots. But strawberries are, and citrus fruits are sour (as is rhubarb, hence the classification above). There are some grey areas: how sweet do you have to be? Yams (sweet potatoes) are vegetables to most people; maybe they're not sweet enough. It was actually this question which led me to think to node this today: I was eating butternut squash soup... butternut squash is also pretty sweet. Is it a fruit? Um, maybe.
Well, something to consider. I don't know if that guideline will really hold up, but at least realize that this whole "tomatoes are fruits!" "No, they're vegetables!" argument is ridiculous as it's usually made.