Well, it's a hierarchy of structure. In the "Real World" of the air force, we'll use the air force academy as an example. There is 1 WING. That is basically all 4400+/- cadets at the academy. Well, it's hard to manage 4400 people, so they are broken up into 4 groups. We'll call them group 1, group2, group3, and group 4. Basically, 1100 cadets in each group. Well, that's better organization for those at the TOP of the food chain, (WING) but for those at the group level, it's still a little difficult to manage. So, each of these GROUPS are then broken up into 10 SQUADRONS. Basically 110 cadets in each squadron. Each of these levels has a person, and staff, in charge. I.e. there is a WING commander and his/her staff. There is a GROUP commander and his/her staff. Same with the squadron. And you report to those above you. e.g. squadron commander reports to the group commander, who reports to the wing commander. And, the squadron is broken up into smaller units called flights. And the flights are broken down further to elements.
Now, during basic training, it's only 1 class of around 1300 students. So, there's really no need for such a hierarchy. So, they basically start at the SQUADRON level. There are 10 squadrons (A-J). In those 10 squadrons, they have 4 flights. Those numbers are cumulative. E.g. A1, A2, A3, A4, B5, B6, B7, B8, C9, C10..... G25..... J40. Now, WHY this numbering system. Well, remember when I used the example of the academy (Post BCT) where there's a wing, group, squadron.... Well, 4 groups with 10 squadrons each, is 40 squadrons. So, your son/daughter in G25 will be in squadron #25 when s/he graduates BCT and becomes a cadet. That is the 3rd group of the wing, which is made up of squadrons 21-30.
As to "What do they do"? That depends. An element (8 people+/-) might be in charge of a small detail. A flight (36 people +/-) might be doing something that requires more people. The squadron (140 people +/-) do other things. Wing, Group, Squadron, Flight, and element are simply different levels of a hierarchy that is more manageable. You are ALWAYS part of something LARGER than yourself. You might be in an element, but that element is part of a flight, which is part of a squadron, which is part of a group, which is part of the wing. Hope I didn't confuse you too much. mike...