You may get different answers on this. I think that sour is the strong acidy lasting taste, such as when you lick a lemon (which is bitter as well). Some people would say that lemon is tart but if an orange is tart, how can a lemon be tart? When milk goes bad, it's sour. Tart is not as strong, not as lasting and is usually mixed with sweet. That's why tart is often used in describing cakes and sweets such as lemon pie: the citric flavor is no longer sour but tart. A Granny Smith apple is tart and so is a grapefruit.

haha - yes, my understanding is quite the opposite to Logan's. I would say that tart is the very citric flavour. Lemons are tart, as are other unripe fruit. Oranges aren't very tart usually. Sour is more general - sour milk, wine that is too dry. 'A tart' is quite different. It is a pie without a top pastry layer.