A live USB is a USB flash drive or a USB external hard disk drive containing a full operating system that can be booted.
Live USBs are closely related to a live CD or DVD, but sometimes have the ability to persistently save settings and permanently install software packages back onto the USB device. Like live CDs, live USBs can be used in embedded systems for system administration, data recovery, or the testing of operating system distributions without committing to a permanent installation on the local hard disk drive.
Many operating systems including Mac OS 9, Mac OS X, Microsoft Windows XP (defunct) Embedded and many of the Linux distributions as well as BSD distributions can also be used from a USB flash drive. Windows 8 can also run from a USB drive with the version Windows To Go.