http://petapixel.com/2014/09/30/your-lens-aperture-might-be-lying-to-you-or-the-difference-between-f-stops-and-t-stops/

Understanding Lenses: Aperture, F-Stops & T-Stops

 

F-stops are the theoretical amount of light transmitted by the lens; t-stops, the actual amount. The difference is about 1/3 stop, often more with zooms.

f-stop is the measurement of the opening (aperture) of the lens in relation to its focal length (the distance between the lens and the sensor).  The math is focal length / lens diameter.
It mainly controls depth of field, given a known amount of light.

https://www.scantips.com/lights/fstop2.html

 

The smaller f-stop (larger aperture) the more depth of field and light.

Note that the numbers in an aperture—f/2.8, f/8—signify a certain amount of light, but that doesn’t necessarily mean that’s directly how much light is getting to your sensor.

T stop on the other hand is the measurement of how much light passes through aforementioned opening and actually makes it to the sensor. There is no such a lens which does not steal some light on the way to the sensor.
In short, is the corrected f-stop number you want to collect, based on the amount of light reaching the sensor after bouncing through all the lenses, to know exactly what is making it to film. The smaller, the more light.

http://www.dxomark.com/Lenses/Ratings/Optical-Metric-Scores

Note that exposure stop is a measurement of sensibility to light not of lens capabilities.

http://digital-photography-school.com/how-to-improve-your-long-exposure-with-photo-stacking/