Usually, to save money, you have to give something up. The trick is making it something you don’t much mind giving up. With “masked” or “opaque” booking, you give up the right to know what hotel you’re booking into — until you’ve already paid for it, non-refundably. In return, you pay way less than the regular rate, sometimes little more than half.
You’re told the approximate location of the hotel, its star rating, and maybe some of the features, like a pool or free wifi. It depends on the site — Hotwire, Priceline, and more recently Travelocity all offer booking this way. If you can stand a little suspense, check it out.