The series is a type of aluminum. You have the 4,000 series, the 5,000 series and stuff like that. Basically, if you are using aluminum in an aggressively saline environment, such as an offshore situation, you will need a specific series that will not corrode in that aggressively saline environment.
Alloys use other materials, such as magnesium or lithium, depending on the final use, such as for aerospace purposes or in the construction environment. It all depends on what the end use is when you are looking at the material.