You will her both terms used interchangeably, therefor making it natural to assume that the two represent the same concept.
As for retargeting, it is often used to describe the online display ads that are shown to visitors who came to your website but navigated away without acting. This type of marketing is done using tracking pixels or cookies. These cookies or tracking pixels follow the user around after they leave your website.
Retargeting ads are served on several networks like the Google Display Network, Facebook, and Instagram, which give you the opportunity to reach out to your potential customers on a multitude of websites.
As for remarketing, it usually refers to reaching out to lost visitors via email. This makes it necessary to have your visitors’ email addresses. It does not include the requirement of cookies and tracking pixels.
Why the confusion? The fault lies with Google Ads. Google defines remarketing as the following: Remarketing is a way to connect with people who previously interacted with your website or mobile app. It allows you to strategically position your ads in front of these audiences or reconnect with them as they browse Google or its partner websites, thus helping you increase your brand awareness or remind those audiences to make a purchase or decision.
Google essentially groups retargeting display ads and remarketing emails under the single terminology of “remarketing” and now many marketers follow the same definition.