Casing and finishing nails are similar and differ primarily in the shape of the heads. A finishing nail has a small slightly rounded head just a tad bit bigger than the nail shank. The head is designed to fit into a nail set to be countersunk and the nail hole filled.
A casing nail is often used in exterior applications and is often galvanized. The nail head of a casing nail is tapered and may be set flush or just below the wood surface.