It's quite straightforward. Most frame manufacturers have a series of 'grades' of aluminum -- the grade is determined by the different metals used to create the raw frame material: aluminum 'alloyed' with other metals.
Both Specialized (Merida) and Giant, for example, use 6061 aluminum alloy as a 'base'. Giant's Aluxx = Specialized's A1: pretty light, reasonable tensile strength, and so on. The '6---' designation refers to the alloying of magnesium and silicone to the aluminum. That allows heat treating. Increasing the proportion of magnesium/silicone increases tensile strength, and so allows for less material to be used for equivalent strength -- so, less weight for a frame as strong or stronger than a basic 6061 frame. It also allows for increased amounts of butting/shaping of the tubes, which some claim contributes to ride quality. So A1: strong; reasonably light; very cost effective. M4/M5/E5: stronger, lighter, and more 'manipulated' (shaped/butted), but a little more expensive to produce.
That's about it, really. Some manufacturers use 7--- series aluminum: the alloying agent is zinc. Very, very strong but rather more susceptible to stress/corrosion cracking than are 6--- series alloys.