Resume from Japanese Wikipedia:
"The idolm@ster is an "idol production simulation" arcade game from Namco (currently Bandai Namco Games).
The player takes the role of a newly employed pop idol manager, chooses 1–3 pop idol candidates to help, and the goal of the game is to get them to be as popular as possible (number of fans). The arcade machines all formed a network and there were rankings with which to compete with players around the country.
The game brags cel-shaded full 3D, full voiced characters, customizable outfits and music, and the game became so popular that a number of music CDs, drama CDs, merchandise, and anime were produced, in the end created the brand "Project im@s" as the developper of the project (the arcade game credited "AMCG (Advanced Media Creation Girls) as the developper, and their logo still sits on the Xbox360 port).
The game was ported to Xbox360, PSP (Idolm@ster SP), Nintendo DS (Idolm@ster Dearly Stars). Due to hardware differences and game time reasons, there are certain game play differences between the various releases, but they are essentially the same game.
The Xbox360 port took the Special Prize in the Japan Game Awards of 2007.
The Xbox port was sequelled by the spinoff "Idol m@ster Live for you"."
Source: Wikipedia, "THE_IDOLM@STER," available under the CC-BY-SA License.