It is some kind of buying service that tracks the selling prices of cars and shows you the best, average and bad prices, then provides you with a list of dealers and the prices they offer
I went to Overstock | Car Buying Home Page and got great prices there - it definitely helped move the process along - though ultimately I bought from a broker.
truecar is great to find out what dealer incentives there are and what the holdback amount is so you can use that in negotiating your deal.
Another great site is carwoo.com