The technology exists to show your ads exclusively to those consumers that had come to your site or SEM campaign prior. The tactic is called Remarketing or Retargeting. Remarketing campaigns are extremely efficient and highly profitable, capturing conversions/sales that may have been lost without the remarketing effort.
Remarketing is a targeting strategy that places a cookie on the computer of a consumer that has browsed your website or clicked on your SEM campaign. These consumers are then served ads for your products and services through the top ad-serve networks. The consumer that may have been undecided is then presented with the opportunity to reconsider their decision.