Your camera’s lens works by refracting the light from what it can ‘see’ and converging it at a single point, the focal point, to create an image. Just how much the lens is able to ‘see’ depends on its focal length.
The distance between the lens and the imaging sensor (or film, if you’re old school) is what determines focal length. It’s measured in millimetres.* A shorter focal length means a wider angle of view, so the lens ‘sees’ more. These are called, somewhat unoriginally, wide-angle lenses. By the same token, increase the focal length and the angle of view decreases and pulls you closer to your subject. Lenses with larger focal lengths, roughly over 135mm, are known as telephoto lenses. Those in between, around the 50mm mark, are ‘normal’ lenses. [read]