Many people are familiar with measuring the f-stop on your film or video camera. What the f-stop measures is the relation between the focal length and the aperture. The f-stop does not, however, take into account how much light may be lost as it passes through the lens itself. Since each lens is different and reflects/bends light uniquely, there is no universal amount that will show how much light is lost. T-stops are a reading that interpret the loss of light through the lens.
The idea of the t-stop is that its base is that of a perfect lens. This perfect camera lens will, theoretically, lose no amount of light as it is contorted. Therefore the the t-stop measures what the lens would pick up if it was not changing the intensity of light. If this perfect lens was in place then the t-stop and the f-stop would share the exact same reading. On different lenses the t-stop and the f-stop readings are going to be different. The amount of difference here is going to be relative to the type of lens that you are using. The type of lens will affect the f-stop reading relative to the t-stop, but only the change in actual locational light will change the t-stop.
There is no exact rule on how film and video lenses are going to be marked for commercial sale. Some film and video lenses include both f-stops and t-stops on them, and others have one or the other. It is best to learn how to read and interpret both f-stops and t-stops so you can be more universal in your motion camera use. You can, however, expect that T-stops are going to be less universally known about and therefore less accounted for on many sets. This is largely a term and measurement coming out of the more traditional days of film and with the newer HD cameras people do not always account for them. This may change as DSLR video shooting has made the standardization of multiple lenses and how they deal with light an important thing for even lower level shooters. For a long time, prosumer and lower end commercial cameras were heading in the direction of single lenses. This lowered the overall understanding of things like T-stops, but this is changing now that most cinematographers are dealing with at least three lenses at any given shoot.
It is best to begin to know your lenses and what affect they have on the over all light. Prime lenses have almost no difference between the f-stop and the t-stop, while zoom lenses can have a great deal. This is really part of the issue with zoom lenses in a larger sense, and the reason that fixed lenses are still going to be the primary necessity on most standardized production using coverage. This is especially true with DSLRs as it will require you adding grain with the ISO because of the light loss. It is best to get a visual of exactly how a particular lens compares the f-stop to the t-stop. It is always best to use an intrinsic visual reading to find out how you will capture the image.