USB mass storage device class

A USB thumb drive and its cap, next to a 100 millimeter ruler for scale
USB flash drives typically implement the USB mass storage device class.

The USB mass storage device class (also known as USB MSC or UMS) is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.