In case you've literally been living under a rock sans wifi for the past six months, you're fully caught up on Bruce Jenner's transition. Basically, he's decided to live as a woman and has undergone various facial surgeries and hormone therapy. He hasn't done sexual reassignment surgery yet because he's not interested in dating.
He's already revealed part of his closet and his favorite LBD, but this will be the first time that the world see's Bruce's “her.” He'll also presumably release her name at this point (Kruce??). Annie Leibovitz is photographing the cover, so you can only imagine Kim and Kendall's jealousy.