Ads are served to people who have previously interacted with a brand, for example through visiting a website, social media profile or searching. In Google, retargeting is known as remarketing. This is a specific name of their programme which has been adopted, more generally to describe all types of ‘reminder ads that follow you about as people often describe them.

This visual from Venn Digital nicely summarizes the options for using online media to target across the customer lifecycle.