Have you ever "burned" a CD-R or DVD-R disc? Have you ever created an ISO image of a CD or DVD or USB drive or hard drive?

Burning a CD-R/DVD-R writes data to optical media using burning software (and, for bootable media, writing an ISO image so the disc is bootable). An ISO image is a single-file, sector-by-sector copy of an optical disc's file system; you can create one from a CD/DVD (rip to ISO) or author one, and write it back to make an identical disc. The same imaging concept applies to USB drives and hard drives—creating a raw/disk image (e.g., with tools like dd, or imaging utilities) for backup, cloning, or creating bootable installation media. Writing an ISO to a USB stick (rather than burning a disc) is now the common way to make bootable OS installers.