The 4 digit system is for wrought alloys. The four digit system with decimal point is for cast alloys.
1xxx, 99% min pure aluminum
2xxx, Cu major alloying element
3xxx, Mn
4xxx, Si
5xxx, Mg
6xxx, Mg and Si
7xxx, Zn
8xxx, other element
There are some exceptions to the rule that for 2xxx through 7xxx, the alloy group is determined by the alloying element present in the greatest mean percentage, but it is a good rule of thumb.
6061 alloy Al-0.6Si-0.28Mn-1Mg-0.2Cr (in wt %)
7075 alloy is Al-1.6Cu-2.5Mg-.23Cr-5.6Zn.
T6 heat treatment is solution treated, quenched, and artificially aged.
T7 is solution treated, quenched, and stabilized.
Solution treatment -- the alloy is heated into the single phase field and held for enough time for any second phase to dissolve and disperse into the solid solution.
Quenched -- rapidly cooled to avoid second phase precipitation and retain the solid solution. Goal is to produce a supersaturated solid solution that is then ready for precipitation hardening.
Artificial aging -- heat the alloy for precipitation hardening.
Stabilized -- overaging that trades off some strength to improve some other aspect such as dimensional stability or corrosion resistance.