Do not confuse the two!
With Retargeting you can serve ads, based on a prior ad that already has been served. The retargeting strategy/campaign be based on a location where the ad is served, a click, time spend on ad, video completion rate and much more metrics.
With Remarketing you can serve ads to devices/consumers that have previously visited your site. Site visitors are considered ‘warm leads’ and serving ads to them is considered a campaign strategy with a strong ROI.