IMG4DC is a tools package containing two utilities to generate selfboot disc images for your Sega Dreamcast: cdi4dc, designed to make Padus DiscJuggler (CDI) images and mds4dc, designed to make Alcohol 120% (MDS/MDF) images.
A selfboot image is a disc image format specially prepared to boot on your Dreamcast. In clear, by burning a selfboot image to a standard blank CD-R, the software containted into this disc image will run onto your Dreamcast, as a legit game disc, without any modchip or other hacking device.
To use this software package, you must have mkisofs from the cdrtools project. This tool will create the ISO image needed by img4dc.
If you want to learn more by the selfboot image format, click here: http://dcemulation.org/?title=Selfboot_formats