A USB flash drive is essentially NAND-type flash memory integrated with a USB 1.1 or 2.0 interface. It is a small, lightweight, removable and rewritable data storage device of up to 64 GB, the most popular retail sizes being perhaps 512MB (as of 2006). The flash drive is a very fast memory device that is more reliable than a floppy drive. Flash drives use the USB mass storage standard for removable storage devices. To use such a device, your operating system must have driver support for both USB mass storage plus the file system used on the flash drive. All versions of Mac OS X support USB Mass Storage devices natively. Microsoft Windows XP (though not some early versions), 2000 and ME shipped with native support for USB Mass Storage devices, but any previous Windows version requires a driver that is usually available from the manufacturer. All versions of Linux that support USB and SCSI storage support USB flash drives, although some desktop environments assume that the drive is partitioned (not a safe assumption). Some recent USB flash drives act as two drives - as a removable disk device (the actual drive itself), and as a USB floppy drive (again, as the actual drive itself, but as another drive in Windows). Normally, the drivers for the removable disk device would be located on the floppy drive (one portion of the removable disk device), for operating systems that cannot find the driver for the drive natively.

USB flash drives are also known as "pen drives", "chip sticks" (though very uncommonly), "thumb drives", "flash drives", "USB keys", and a wide variety of other names. They are also sometimes called memory sticks, which can lead to confusion because memory stick happens to be a Sony trademark describing their proprietary memory card system.