Priceline - There are three ways to book hotels room:
Priceline as a normal
travel agent (like Expedia and Orbitz). Go to the main website, enter destination and dates, and you're shown Hotel Names, Rates, and hotel quality (stars).
Priceline Express is similar to Hotwire in that you see room availability listed as hotel quality (stars) and rates. You pick the hotel quality your comfortable with (I usually stick with 3*+) and a reasonable rate. You don't find out the hotel name until after the transaction. There are a couple of websites that have a good database of past Hotwire/Priceline deals, so with research, this is usually a good way to go.
Name Your Own Price is Priceline's original business model, where you select destination, dates, and hotel quality THEN make an offer for the rate. You won't know what the hotel name is until your bid is accepted. Again, with research, one could narrow down what might be in the inventory and what bids had been accepted in the past. For example, Priceline Express might offer a 3.5 star for $80/night but you might be able to "win" the same room for $65/night with a bid.
BUT this takes a lot of research and you may have to bid several times to find the lowest accepted rate.
Hotwire is generally the same as Priceline Express. You enter destination and dates, and a list with hotel quality and rates are displayed. Rates between Hotwire and Priceline Express are close but I think Priceline is a little cheaper ($5) on average.
I recommend visiting either BetterBidding or BiddingForTravel websites for more in depth information.
HTH, Steve