USB drives are storage drives connected by USB (Universal Serial Bus) to the computer. A USB drive -- also known as a flash drive or keychain drive -- is a plug-and-play portable storage device that uses flash memory and is lightweight enough to attach to a key chain.
Above the OS level, work with these drives does not differ much from the work with the fixed drives: they can be partitioned and partitions can be formatted OS-supported filesystems.
Such drives can also be bootable. Developers may use this capability to test new operating system without installing it, or to gain access to non-encrypted fixed hard drive if the main operating system does not boot any longer.