Magnification: A 1.4x extender multiplies your focal length by 1.4, while a 2x extender doubles it (for example, turning a 300mm lens into a 420mm or 600mm lens).
Light Loss: A 1.4x extender reduces your maximum aperture by 1 stop, and a 2x extender reduces it by 2 stops.
Compatibility: They are built primarily for select L-series telephoto and zoom lenses and do not fit crop-sensor EF-S lenses.