The different Aluminium designations have to do with their alloying constituents. The first number is the main element i.e. 6xxx is Silicion, 7xxx is Zinc (cant rememeber what 2xxx is - maybe Copper). The rest of the numbers are the identification number given to it when the composition is lodged with people who decide these things (I should know this).
Different alloys have different properties i.e. Weldability, ductility, malleability, etc.. and as such are suited to different tasks eg 7075 high strength, good fatigue life, slightly brittle - good for cranks, ok for hubs. 2014 good strength, high fatigue life , hard to work - good for hubs, difficut to make into rims.
The T* designation is the heat treatment level, T0 - fully annealed, T4 - half hard, T6 - 3/4 hard.
For more specific alloy information try www.matweb.com