I simply copied the following article from Wikipedia.
A gravure idol (グラビアアイドル, gurabia aidoru), often abbreviated to gradol (グラドル, guradoru), is a Japanese female model who primarily models on magazines, especially those marketed to men, photobooks or DVDs. Gravure idols, in most cases, emphasize their sexual attractiveness and often model in swimsuits.
“Gravure” (グラビア) is Wasei-eigo (Japan-made English) term from “Rotogravure” a type of intaglio printing process that was once a staple of newspaper photo features. The rotogravure process is still used for commercial printing of magazines, postcards, and cardboard product packaging.
Gravure idols appear in a wide range of photography styles and genres. Their photos are largely aimed at male audiences. Contrary to popular foreign belief, gravure idols never appear fully nude in their photos but their poses often can be labeled as semi-provocative.
Gravure models may be of any age but usually range from their late teens to their mid-twenties. In addition to appearing in mainstream magazines, gravure idols often release their own professional photobooks and DVDs for their fans. Many popular female idols in Japan launched their careers by starting out as gravure idols.
Gravure models are commonly photographed wearing bikinis or lingerie, but may also appear wearing schoolgirl uniforms, Japanese street fashion, and kimonos.
In Chinese, Japanese Gravure Idols are referred to as 日本女星.