Aha, I'm a Jpop fan :D I love Kpop, but I'm more into Jpop.
Well..The best selling girlband is AKB48. There's 48 of them...O-O In about 2000 it was Morning Musume. Morning Musume are still ranked at Japans 7th top artist of all time, although they don't sell much anymore. They're been going tfor 14 years, but they change members(not all at once, they add members and sometimes member graduate)
Other girl groups are:
Perfume
Berryz Koubou
C-ute
S/mileage
Buono!
Mano Erina
Kikkawa Yuu
Ayumi Hamasaki - She sells Millions
Hikaru Utada - She's Japans best selling artist at only 23. She's currently on Haitus because she said she wants to 'live life normally' :( Awww Hikki!
YUI
Scandal
Tokyo girls' style
Momoiro clover
As for Boybands, Arashi is the best selling. Arashi and AKB48 dominated the top 10 best selling singles of 2010. Other bands like Kat-un, Hey! Say JUMP are also popular.
Kpop is very different to Jpop. Kpop is really simmilar to American music. Jpop is like wow. Sometimes the cuteness can kill you =[ They're not 'sexy' they're 'sekushiii!' Which is how they say it in japan XD
Morning Musume's new single =D
http://www.youtube.com/watch?v=uPGgHeBcMyg&feature=related
AKB can be very very sexual...not sooo much here though. Skip to 2mins.
http://www.youtube.com/watch?v=SwXtaahxjls