F-Stop or f-number is the ratio of aperture diameter to focal length of a lens, giving an indicator of the light-gathering power of the lens.
The larger the f-number, the smaller the lens opening, i.e. a number of f2.8 indicates a larger aperture than f4.
A T-stop is a lens marking which indicates the true light transmission of the lens at a given aperture as opposed to the approximate light transmission indicated by the conventional f-stop marking. Often found on cinema camera lenses.