From Knoppix Documentation Wiki
The el torito bootable cdrom format using the syslinux boot loading process uses floppy emulation to boot. The emulation process uses the boot.img as the filesystem image of a floppy disk.
This info is given to the mkisofs program during the building of the iso image for burning to the cdrom. During the boot process, the boot.img is used as if it were from a floppy drive.
Located on that image is the ldlinux.sys driver which gets executed.
The syslinux.cfg is a text file that is configuration for ldlinux.sys. It specifies files to display with various key-presses, and kernel files to load with their parameters such as ramdisk images. At this point in the booting, a user will have screen with different boot choices from which to pick.
With the Knoppix cd, when the user picks one of those choices, either by waiting for the timeout period to expire or by typing in a selection such as 'knoppix lang=us', the kernel image and initrd image as specified by that entry in the syslinux.cfg configuration file, will be loaded into memory and the kernel is executed. The kernel will use the initrd as the initial root file- system. This root fs will be the miniroot.gz file on the boot.img file with Knoppix.
See also : Faq Booting.