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.
Gravure idols are Japanese models who often pose in bikinis and other provocative clothing for photo spreads in magazines aimed largely at men.
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.
Although they are portrayed as idols, they are also a symbol of sexuality, and often seen as the ideal woman.
In the 1990s, instead of being marketed as people who lived better and were better than average, idols became people who just happened to have a little something to become popular. Where the tastes of past idols had to be saccharine, it was now acceptable for an idol to simply love eating ramen or to display something other than a smile, to lament having got a little out of shape or to admit to shopping around for lower prices. Idols also became a fixture in countless anime by singing opening or ending songs that have little relevance to the anime itself. Some experimented with being seiyu, and seiyu themselves became somewhat like idols, becoming increasingly popular. Even today, some are still involved with the video game industry, though they are not always entirely successful.