A bootable USB is a portable storage device, like a flash drive, that contains a bootable operating system or software. This allows a computer to start up and run the operating system directly from the USB instead of the internal hard drive. Bootable USBs are commonly used for installing operating systems such as Windows or Linux. Creating a bootable USB typically involves using special software to transfer the operating system files onto the USB drive in a way that makes it bootable. This method is often preferred for its convenience and speed, especially when compared to using CDs or DVDs for installations.