Base - Base mouldings are placed along the wall at the floor. They should be chosen to work in harmony with the casing. Base mouldings are generally thinner than the casing.
Casing - Casing defines 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 (or other wall surfaces) and the door or window frame. Casings are generally thicker than base mouldings.
Stop - Stop is attached to the door jamb on both sides and at the top. It is where the door comes to a rest when it is closed, stopping the door from moving any futher and covering the gap that would otherwise appear between the door and tha jambs. Stop is also used as perimeter trim on the inside of vinyl and other replacement windows.