Focal length refers to the distance between the lens and the camera's image sensor when the lens is focused at infinity. It is typically measured in millimeters (mm) and determines the angle of view and magnification of a lens. A shorter focal length (e.g. 18mm) provides a wider angle of view and is suitable for landscapes and architecture, while a longer focal length (e.g. 200mm) provides a narrower angle of view and is ideal for portraits and wildlife.