Remarketing (or Retargeting) falls into the display marketing bracket, but with differing targeting methods from traditional Display Network methods.
Leaving cookies with users who visit your site allows Google to serve your ads to these same users when they visit any site in the Google Display Network.
Not only does this allow you to reengage with users who show an interest in your brand but fail to convert, it allows us to group prospective customers based on on-site behaviour, and market to them individually.
