Actually, there is a big very difference. When you sauté the onions you soften them and bring out their flavor, but when you caramelize them you slowly (about 30 min total) cook them over low temperature which brings out the natural sugars and eventually that sugar caramelizes as the onions brown some people even add sugar to speed up the problem.
It is almost like a reduction using an onion instead of a liquid.
A sautéed onion still tastes much like an onion while a caramelized onion has onion flavor but is very sweet, a completely different beast.
