The terms 'remarketing' and 'retargeting' are most easily differentiated in terms of marketing goals: retargeting serves the purpose of bringing visitors back to your website and encouraging them to make their first purchase, whereas remarketing intends to re-engage your existing customers and incentivise loyalty and repeat purchases.
These two goals are often also used in conjunction, since often, remarketing to existing customers allows partners to gain insights into new audiences that they can feed into the strategy of their retarageting campaigns, or of prospecting campaigns (looking for entirely new audiences).
Remarketing primarily uses email campaigns to re-engage past customers. By comparison, retargeting campaigns tend to use paid display advertising to re-engage audiences who have only visited your website or engaged with your social media posts, but not yet converted.