A hybrid disc is a CD-ROM that has multiple file systems, so that it can be used on various system software, for example both Apple Macintosh and Microsoft Windows based operating systems.
A hybrid disc has multiple file systems installed on it, typically ISO 9660 and HFS+ (or HFS on older discs).
The reason for the format is primarily that ISO 9660 has severe restrictions placed on filenames (initially only 8 characters per file, and a depths of 3 directories, similar to the Microsoft FAT filesystem) and lacks key structures present in Apple Computer's HFS and HFS+ file systems. Another key factor is that ISO 9660 does not support resource forks, which is critical to Mac OS' software design (Mac OS X has removed much of the emphasis/need for resource forks in application design).
Companies that released products for both DOS (later Windows) and Mac OS (later Mac OS X) could release a CD containing software for both, natively readable on either system. Data files can even be shared by both partitions, while keeping the platform specific data separate. In a true (or shared) hybrid HFS filesystem, files common to both the ISO 9660 and HFS partitions are stored only once, with the ISO 9660 partition pointing to file content in the HFS area (or vice versa). Blizzard Entertainment has released most of their computer games on hybrid CDs. By default, Mac OS 9 and Mac OS X burn hybrid discs.
A hybrid disc has an ISO 9660 primary volume descriptor, which makes it a valid ISO 9660 disc, and an Apple partition. It may also have an Apple partition map, although this is not necessary. The ISO 9660 portion of the disc can co-exist with an Apple partition because the header areas which define the contents of the disc are located in different places. The ISO 9660 primary volume descriptor begins 32,768 bytes (32KB) into the disc. If present, an Apple partition map begins 512 bytes into the disc; if there is no partition map, the header for an Apple HFS partition (known as a Master Directory Block, or MDB) begins 1,024 bytes into the disc.