Camera Lens

I just need to know something very simple, what does the camera lens value represents and how do we convert this to a field of view and back to this camera lens value. It looks like nowhere in the wiki manual this value is explained.

I always assumed it approximated focal length so 35 is reasonably “wide angle”, 50-70 is “normal” and greater than 70 is “telephoto”. I don’t know if the numbers translate exactly to length in millimetres but the general result is as expected.