Japan is famous for its idol groups. AKB48 is one of many… but maybe the biggest in Japan.
The AKB stands for ‘Akihabara‘ where their theatre is.
Yes. They have a theatre and do shows almost every day. That was the idea behind them – create a group that is accesible for their fans to see anytime.
And they do have a lot of ‘fan-service’. See their video for ‘Heavy Rotation’:
And the 48 stands for… well, who knows? They currently have around 90 members.
The members are national stars – appearing in television dramas, movies and so on.
If you hear of any other groups with a similar name, like JKT48, NMB48 or SKE48, these are sister groups of AKB48 for their areas.
They are a cultural phenomenon and if you want more information check out their giant wikipedia entry.