There isn't exactly a clear definitional difference between roasting and baking. For chefs and foodies, it's kinda like pornography, you just know it when you see it.
Nevertheless, roasting is usually higher heat than baking with the former being over 400 degrees Fahrenheit and baking using being between 350 and 375 degrees Fahrenheit. Roasting usually involves cooking something in an oven with high, dry heat until it develops a caramelized/brown crust and moist, tender interior. You are not changing what it is, just cooking it in an oven.
In contrast, baking is usually described as a transformational process where you turn several ingredients into a new singular item using an oven, eg. Turning dough into bread or batter into cakes, or potatoes, milk, cream, and cheese into a gratin, or ground beef and various fillers into meatloaf.
Martha Stewart has a good article on it:
How Roasting Is Different From Baking—and Why It Matters.