With Google Display Ads, you set the price you want to pay when your ad is displayed. When there's an available ad spot, we compare your price to the prices set by other advertisers, and the winner gets the spot. We call this the auction.
After each auction, Google adjusts the winning price so it's the lowest amount needed to win the spot. If the winning price is 55 cents and the second-place price was 43 cents, the winner pays only 44 cents for the spot, not the full 55. So you can always bid with confidence that you won't overpay.
Setting Your Price
When do you pay? Only when someone clicks on your ad
Why choose it? CPC is perfect if your goal is to get more traffic to your website. You pay only when someone clicks your ad to visit your site. Set your own bids or choose automatic CPC bidding, where you let Google technology find you the most clicks possible within your budget.
Cost-per-thousand impressions (CPM)
When do you pay? Each time your ad appears
Why choose it? If you want your ad to be seen as often as possible by your target audience, and don't care so much about clicks, CPM is a great choice. You bid per one thousand views, so a CPM of $10.00 means you pay one cent each time your ad is displayed ($.01 x 1,000 = $10.00).
When do you pay? When your ad leads to a sale or conversion on your website
Why choose it? If your end goal is sales, email sign-ups, or other online transactions, CPA may be for you. Set a target CPA -- the amount you want to pay for each conversion -- and the Conversion Optimizer takes care of the rest, putting your ads in the auctions most likely to lead to conversions.
Get more for less with the Conversion Optimizer
The Conversion Optimizer uses smart technology to find you more conversions at a lower cost. Google analyzes your own conversion tracking data to put your ad in the auctions most likely to win you profitable clicks.
Conversion Optimizer campaigns get, on average, 21% more conversions with a 14%
lower CPA. 
Display Campaign Optimizer
The Display Campaign Optimizer handles both targeting and bidding, so you don't have to lift a finger. You set a target cost-per-acquisition (CPA), and the Display Campaign Optimizer finds the best placements and the right bids to win you the most conversions.
 Average impact based on the performance of Conversion Optimizer campaigns compared to a control set of campaigns. Actual impact will vary by campaign (and a small number of advertisers could conceivably perform better without the Conversion Optimizer).