A Live USB is a portable storage device, like a USB flash drive, that contains a complete operating system. It allows users to boot and run the operating system directly from the USB without needing to install it on a computer's hard drive. This is useful for testing new operating systems or troubleshooting issues without altering the existing system. Typically, a Live USB can be created using various tools, such as Rufus or UNetbootin, which copy the operating system files onto the USB. Many popular operating systems, including Linux distributions like Ubuntu and Fedora, offer Live USB versions for easy access and use.