Visually, a card often has a header and/or footer and is generally embeddable into a standard article body (e.g. Twitter's embeddable tweet cards). Content-wise

Visually, a card often has a header and/or footer and is generally embeddable into a standard article body (e.g. Twitter's embeddable tweet cards). Content-wise, a card serves a very specific semantic purpose: presenting a self-contained block of data to a user in a standalone way. The metaphor employed here is something like a trading card or baseball card, such that a block of data or metadata about a particular subject is called out in a self-contained way that is easily understandable and digestable.