What is an ‘NFC smartphone case’ and how does it work?

Near-field communication (NFC) smartphone cases have gone viral on social media over the last few months. These cases enable users to express themselves by displaying a photo or graphic of their choice. NFC smartphone cases are possibly some of the most unique smartphone cases, where NFC is just a part of it, while a coloured e-ink display is actually the heart and soul of an NFC case.