Visual kei is a style of stage appearance that used to go hand-in-hand with J-rock, but now seems to have crossed somewhat to J-pop as well. The style includes crazy hair (not giant fro crazy, more like spiked and colored, think Wayne Static of Static-X with red or blue hair), goth or androgynous clothing, and elaborate makeup.
Now, of course, not all visual kei bands will have all these traits. And don't go comparing them to KISS, if you need a name under an American label to compare it to, think more along the lines of Marilyn Manson or Switchblade Symphony. Some bands that were or are visual kei include X Japan, Dir en Grey, L'arc~en~Ciel, Luna Sea and Malice Mizer. If you're still confused just do a Google image search for one of the bands listed above.