They are different lighting strips. T stands for tube and the number is the diameter of the tube in eighths of an inch.
T8 is the original aquarium lighting. It is 8 eighths of an inch (1 inch) thick. You can get different tubes depending on the type of tank you have.
T5 is the newer lighting tube it is thinner at 5 eighths of an inch but far more efficient. This means that the tube produces more light than a t8, which is better if you are growing corals or plants. Again you can get different lights depending on the type of tank. The t5 is more expensive than T8 but that is to be expected as a newer product.
Hope this helps