In a great article on the Adobe Creative Cloud blog author Nick Babich explains what cards mean to the UI designer, and reviews three popular card-based services.
Web and mobile apps are moving away from pages towards completely personalized experiences. These new experiences are built on an aggregation of many individual pieces of content. The way this content is now being presented is in the form of cards, the new and trendy creative concept.
Source: XD Essentials: Card-Based User Interfaces | Creative Cloud blog by Adobe