It's a clothing style and musical genre in Japan. Quite popular stateside, but is actually pretty underground in Japan except for a few mainstream bands like X[Japan] (defunct), LunaSea (once on hiatus but not anymore), Dir en Grey (abandoned the Visual-Kei style out of disgust), and Pierrot (defunct). It's kinda like a mishmash of glam rock, hair metal, shock rock, kabuki and over-the-top theatrics.