F-stop (most often used in photography) refers to the ratio of aperture (the size of the opening at the front of the lens that lets through light) to focal length (measured in mm), regardless of actual light transmitted. But all lenses absorb a part of the light passingt hrough them (as light gets refracted and passes from the front of the lens tothe film plane, and the amount being absorbed varies from lens to lens).
T-Stop is used as a standard for cinematography. This refers not only to the size of the opening of the lens, but in fact to the amount of light that finally reaches the sensor or film plane. Since all lenses absorb some light, the T-number of any given aperture on a lens will always be greater (less light transmission) than the f-number. For example, a lens with f-stop 1.2 can have a t-stop of 1.3,meaning a small portion of the transmitted light has been absorbed by the lens' glass elements.
The better the quality of the lens, the closer the T-stop will measure to theF-stop.