Baking Versus Roasting
Your recipe for roast pork loin says to roast in a 350-degree F. oven. Your recipe for yellow butter cake says to bake it in a 350 degrees F. oven. For either recipe, you open the oven and put your food in. So, is there a difference between baking and roasting?
The short answer is "No."
But it isn't as simple as all that. Baking and roasting are both dry-heat cooking methods. This means that heat is not transferred through a liquid medium during cooking. In modern times, we assume that baking and roasting occurs in ovens.
Joy of Cooking defines roasting as a specialized type of baking. Roasting is almost always done in an open pan; the food to be roasted is uncovered. When roasting meat, you often place it on a rack so it doesn't sit in its own juices as it roasts. Instead, the rack serves as a suspension system whereby the meat is "suspended" in the oven over a pan (shades of spit roasting in days of yore).
There also seems to be a convention associated with the terms "bake" and "roast." Although the two identify almost identical cooking techniques, in the modern kitchen anyway, "baking" is most generally associated with bread, cakes, pies, and casseroles, while "roasting" is what you do to meat, vegetables, or garlic.
Roasting often starts at a higher temperature, creating a "crust" outside of what is being roasted. Then, the temperature is reduced for the remainder of the cooking time. This is also the case when baking pate a choux (for cream puffs or éclairs) and some bread. The identical cooking process (high temperature reduced to a lower temperature) is employed in these similar cases for different reasons.
In the roasting example, you're trying to encourage exterior browning and caramelization of the target food before decreasing the heat and finishing gently. In the baking example, you need an initial burst of intense heat to encourage an expansion of air to make the pate a choux puff up or to encourage optimum oven-spring in the bread (the yeasts' last hoorah).
Then, the temperature is reduced to set and dry the structure of the pate a choux and the bread.
What's the Difference?
So, while roasting and baking are almost identical methods of dry heat cooking, the terms roasting and baking apply to two different kinds of foods. You generally roast food that has structure already, solid foods such as meats and vegetables. You generally bake foods that only have a little structure once baked: cakes, bread, pies, casseroles, creme brulee, etc.
In other words, you bake leavened items - items that "puff up" or "rise" during the cooking process. Aside from just "cooking" the food, baking aims to create steam or expand air pockets within the target food.
Most foods we roast contain less "empty space" than foods we bake. This is because these foods are, by and large, already solid. The primary goal of roasting then becomes transferring heat from the surface of the food to the interior at a regulated pace to ensure crusty goodness outside and juicy, tender doneness inside.