T5 is simply a collective term for a narrow-diameter fluorescent light tube. The 5 simply means the tube's diameter is 5/8". Standard 1" tubes are also called T8.

T5 lighting

It is claimed that certain T5s are roughly three to four times more effective than a standard fluorescent bulb of similar wattage. Therefore, they are the ideal lighting system to use for your aquarium.
T5s comes in a variety of colors (temperatures) so the aquarium-keeper can choose a set of colors that best fits his aquarium.

T5s produce a "flatter" light, like that produced by a standard fluorescent, rather than an intense directed spot of light like a metal halide. The light levels throughout the tank are more uniform, but you don't get the natural-looking rippling light effect on your substrate you get from a metal halide. Some reefkeepers reckon they lack the "punch" of metal halides for penetrating deep water, so many use a combination of the two types together. Compared to a standard fluorescent, they're considerably brighter to look at.