Japanese idols are predominantly singers and actors, as well as models. What they all have in common is the "kawaii" (cute) factor, and their pretty faces are plastered all over the mass media, to the delight of guys.
A Japanese idol is a celebrity who achieves widespread popularity and fame in Japan largely by virtue of her looks.
The term 'Japanese idols' usually refers to female performers in their late teens and early twenties who are considered "kawaii" (pretty, cute) and achieve fame through publicity in the mass media.
The idol phenomenon began during the early seventies, reflecting increasingly materialistic and richer Japanese youths. Teenage girls, mostly between 14 and 16, began rising to stardom.
During the yuppie years of the 80s, idols dominated the pop music scene, and this period of time was otherwise known as the Golden Age of Idols in Japan.
Some idols became famous because of their singing ability (quite a rarity), and also because of the usual good-looks and cute factor.
Some idols already had successful television or movie careers, and voice acting created a whole new generation of idols.
Not all idols started out as singers, but plenty came from other industries. Take for example, Reiko Chiba, who became popular among the games playing public because of her usual presence on the Internet and in video games.