Prime Wardrobe items are limited to women's, men's, kids' and baby clothing and accessories sold directly by Amazon.com. You can add anywhere between three and eight items to your box, and once you check out, your selections will arrive between four and six business days (no free two-day shipping here, unfortunately). You have a week to try on the items and decide if you are keeping them without being charged. Once you decide what you want to keep, you can check out in Your Orders. Anything you keep, or that isn't returned to Amazon, is charged to your credit card.
The big difference between Prime Wardrobe and Stitch Fix (and other style-box services) is that you choose what comes to your door, rather than letting a stylist pick out clothes for you. Additionally, there's no fee to use it, unlike other services that charge you if you don't keep a clothing item or accessory. Clearly, it's been a hit with the smaller test pool, which is why Amazon is now expanding the service to all Prime customers.
Update: This article was edited to correct the number of business days it takes to receive a Prime Wardrobe box.