I'm not an expert but I've been a fan of all the '48' world since the end of 2010 so I'll help you but in order to know more stuff about them you should check Stage48.net. That website also has a wiki page with a lot of info which will be useful
AKB48 has some subgroups with the most popular members:
- Not Yet
- French Kiss
- No3b aka No Sleeves
- Watarirouka Hashiritai
- DiVA
There are other subgroups, for example MINT, which songs have been included in some singles or albums but they don't release their own CDs.
Undergirls is how they name the less popular girl of each group (I'm not sure if it's the same in SDN48 because I don't know too much about them). The popular ones are known as Senbatsu members. AKB48 have had 3 senbatsu elections (2009,2010,2011) in which fans could vote for their favourites using a coupon included in some singles. If you are interested in this I can search for a video with english subs that contains a great explanation on the subject.
If you have any other doubts feel free to ask
