Retarget people based on the actions that they have taken on your site
The Google Adwords and Facebook Ads platform let you target visitors that have previously visited any or specific pages on your website. A retargeting campaign gives you the ability to apply specific campaigns (and messages) that are relevant to people that have visited and taken specific actions on your site.
Because people have expressed some form of interest in what you do and you can be very specific with your campaigns, the click-through and conversion rates for retargeted ad campaigns are huge compared to standard advertising to a cold audience.