They are not exactly the same. The image plane is the physical plane where the lens projects the image—on a digital camera, that means the sensor; on a film camera, the film. The focal plane is the plane where the image is in focus. In practical camera use, people sometimes loosely say “focal plane” when referring to the sensor/film plane, but strictly speaking the terms describe different ideas. The image plane is a fixed physical location in the camera, while the focal plane refers to the plane of focus formed by the lens. When the lens is focused correctly, the in-focus image falls on the image plane, so the two coincide for that subject distance. That overlap is why the terms are often confused.