The idea behind Priceline.com is that they auction hotel rooms and flights to the highest bidder, more or less. The flights are often last-minute deals where the airline is trying to fill the last few seats. Hotels may be offering cheaper rooms during off-season periods. Whatever the reasons, you can find these deals at Priceline.
While there are undeniable bargains to be had, you should be aware of the many customer complaints against the company. The main problem is lack of customer service if you need to make any changes. It can be very difficult to get anything changed, either with Priceline or the actual airline or hotel. Just hope you don't have to cancel, because there is a good chance you won't get your money back.
Just be aware that you might be taking a bit of a risk with them. Make sure you double-check all dates and prices before agreeing to anything.