Retargeting is the process of identifying an individual, who has done something you have defined, in order to determine who to target with an ad.
People at Chango came with great summary of 7 types of effective retargeting:
Retargeting off-site events – add new customers to your funnel who have not previously engaged with your site:
Search – Target individuals based on the searches they conducted on Google, Yahoo! and Bing.
Social – Target individuals who consume similar content to your existing customers.
Engagement – Target individuals who have interacted with distributed content (a custom Facebook page, an expandable video ad unit or an online game/application).
Contextual – Target individuals who have visited a partner site that shares a similar interest / topic to your own site.
Retargeting on-site events – help retain and convert existing customers that have already visited your site:
Site – Target individuals based on the specific products viewed, actions taken or actions not taken (such as abandoning a shopping cart).
SEO/SEM – Target individuals based on how they arrived on your site – use the inbound event to better customize messaging.
Email – Target individuals who are interacting with your email programs.