You will find many people here who use Priceline. The
Budget Board is a great place to look, as well. I'd highly suggest visting
www.biddingfortravel.com. I used Priceline often before finding this site, but never knew what I was doing until I found them.
To answer your direct question, to bid you must give them your CC# and be prepared to buy. There are no refunds. If you bid and win, you own it. For that reason, be sure you're going before using Priceline.
Priceline has its place. For Hotels, I find it fantastic, at least in Orlando. My needs for a hotel aren't much, and for $30 to $35, I can get a hotel that would cost me over $50. So I use it extensively for hotels.
For rental cars, I seem to be better on my own. The compact and subcompact prices on Priceline just aren't what I can get with codes and research.
The wildcard is air fare, which many people use Priceline for. I had a bad experience where they changed my flight times (Baltimore, MD to San Jose, CA) significantly on short notice (less than two weeks, if I remember correctly). Kind of threw my schedule off. For that reason, I won't use them for air fare. Plus, anymore you can get decent discounts on air fare without them. Others would disagree.
So the bottom line, before you put in a bid, know what you can get. Read everything on Bidding for Travel. If you follow their rules, they'll actually help you get together a bidding strategy. My rule, for instance, when using Priceline for a hotel: I look at the hotels that are coming up, I figure out which one that's come up the last couple of months I'd least like to stay in. If I can be happy in that hotel for a given price, then I bid. If I get the hotel I want, all the better. Example: The Swan/Dolphin was coming up for resort at around $70 give or take for a while. But others were coming up, too, like the Hyatt DTD. Folks were bidding because they thought they'd get the Swan/Dolphin. Priceline doesn't work that way. You're not bidding for a certain hotel, or a certain flight. You're bidding for a hotel in a given zone, or a flight on a given day. If getting a flight that puts you into Orlando at 9 PM the night you travel will ruin your trip, don't use Priceline.
Hope that helps!
Pat