Here's something they never teach you in Japanese class (unless your teacher is either very cool, or has questionable ethics), yet no reason I can't bring it to you here!
A グラビアアイドル can best be explained by submitting a link and me saying "that's what it means" and you (hopefully) going "ohhhhhh."
So here's the link (no nudity - but probably not good for younger audiences).
If I need to explain further, and I suppose I should since this is a blog for explanations of things that aren't explained simply in other places, then a グラビアアイドル is basically a girl that poses in suggestive ways and comes very near that line of being pornographic. Many of these idols DO pose naked at some point (no, I do not spend time searching for them and downloading their pictures) but their primary claim to fame is typically certain voluptuous features perhaps not typical of Asian DNA. Then they do everything they can to put these wares on display short of revealing the areola.
This isn't a cultural blog specifically, yet if you've never been to Japan you should know that pornography is rampant in this country. In areas where there are lots of bars, there are typically also hostess bars where pictures on the front tempt salarymen inside. These pictures leave nothing out. There ARE laws in Japan against photographing pubic areas without a blur, but everything else is right out there on the street.
This is also true of pornographic magazines in convenience stores. No covers, no plastic bags - children can walk in and start leafing through whichever weekly catches their eye. I do not feel that is socially appropriate, but I don't make the laws here.
If you are planning to travel to Japan with young children, watch them closely in convenience stores and be careful walking on the streets because you never know when one of these life-size グラビアアイドル posters are going to leap out from around the next corner in front of some porn video shop.