Baseboards are placed along the wall at the floor. They should be chosen to work in harmony with your casings to finish and tie the room together. Baseboards are usually thinner than the casing.
Casings define the overall character of a room and are often the most visible part of the trim. They are used primarily to cover the gap between drywall and the door or window frame. Casings are generally thicker than the base mouldings.