This is a good article explaining retargeting/remarketing. We use it regularly on EF & AdWords through Search and Display with a lot of success.
Amplify’d from searchenginewatch.com
If you spend “a lot of money” on your clients’ paid search programs or any other marketing program, this is your opportunity to increase their campaign ROI by making an effort to recapture the missed opportunities a second, third, or umteenth time. In most cases retargeting campaigns outperform search marketing campaigns (in a silo).
See this Amp at http://bit.ly/eQl1lt