A live USB is a portable storage device such as a pen drive, which holds an live operating system in it. The operating system that in loaded into the storage device is ready to boot, it doesn’t require an installation or extraction.
Live USB permits the user to carry their operating system of choice, with their personal data, such as documents, files, music etc… Normally, a Linux distro is installed in the storage device, since it is compatible with majority of the hardware in the market. There is a similar windows version as well, know as Windows To Go.
This is much much more better than a Live CD, since it is much more faster and lets you to travel with your personal data.