A Bay window is formed from several casement window frames projecting out from an outside wall and normally have it’s own foundations allowing you to step into the window area. It could have 90’ square corners for a rectangular shape or less than 90’ for a curved shape. The overall size of the room is increased.
A Bow window is usually a curved shape window projecting out from an outside wall and it is often supported underneath externally. The window will have a deep internal window sill or window seat. The overall size of the room is not increased.