Roasting is a method of cooking that involves dry heat and usually results in the surface of the roasted item carmelizing.
Traditional roasting involves cooking meat or vegetables over an open fire, sometimes on a spit and turning the item to ensure even cooking. The food was usually basted in some sort of juice, fat, oil or lard to retain moisture.
A roast is a food item that has been roasted, such as roast chicken. It can also refer to an uncooked item intended to be roasted, like a rib roast.