Japanese Idol groups are a group of vocalists (from 3 to 32) who sing and/or dance to either live music or pre-recorded music tracks. The musicians remain anonymous either way in most cases.

The term "Idol" defines the group composition, rather than the genre of music they perform, it is safe to say that a high percentage of Japanese Idol music is Jpop. That being said, the musical genres available in idol music are as vast as in regular music, as different groups try hard to distinguish themselves from the crowd, in an effort to get a larger fanbase and more recognition.
Subgenres of Idol music can include Kawaii Metal, Which was made quite famous by one particular Idol group. Then there's alternative Idol & IDOLCORE. I imagine there is IdolHip/hop, And I Know there's Idol Country/rockabilly as well.
Many of these subgenres are test marketed by the larger production groups in an effort to expand their listenership by making sure that there's a genre of music for everyone. The number of Japanese idol groups seems almost infinite. I've put four example videos below. I found it quite difficult to limit myself to that number. ! I feel they are very good examples of how Idol normally appears in video.
Some of the Idol units that I like, are extremely popular, Probably why I found them to begin with. Many Japanese idol groups will get their own individual pages as we move forward. Please send me an email with your favorite idol group to iluvjapanesemusic@gmail.com

KamenJoshi

@onefive

Batten Girls

Manacle

J-Pop, What is Japanese Idol ?