The inventory is called "opaque" because the specific suppliers (i.e. hotel, airline, etc.)
According to TravelClick, the opaque channel accounted for 6% of all hotel reservations for major brands in 2012, up 2% from 2010.
While one has control over the dates and times of a travel itinerary, the downside is these purchases are absolutely non-refundable and non-changeable and, as noted above, the specific hotel or airline is not revealed until after purchase.
Hotwire has a fixed pricing model, where it sells a room at a fixed price with a limited description of a given venue, whereas Priceline offers both a similar fixed pricing model and a bidding model where travelers bid for a hotel room from among a group of hotels of a given star rating and location.
Typically hotel deals are greater than airline discounts on opaque travel sites, namely because airlines have limited seating and also take monetary cuts when publishing discounted fares, whereas a hotel sells to opaque sites to fill empty rooms.