Wikipedia
MSCDEX or Microsoft MS-DOS CD-ROM Extensions is a software program produced by Microsoft and included with MS-DOS 6.x and certain versions of Microsoft Windows. Earlier versions of MSCDEX were installable add-ons beginning with MSDOS 3.1.
It is a driver executable which allows DOS programs to recognize, read, and control CD-ROMs using the ISO 9660 file system. This requires the previous loading of an appropriate CD-ROM device driver (example: OAKCDROM.SYS), usually from CONFIG.SYS. The program was used up until Windows 95 when it was replaced by the 32-bit version CDFS.
The final version of the MSCDEX program was 2.96, included with Windows 95 and used when creating bootable floppy disks with CD-ROM support.
A cloaked variant of MSCDEX was provided as part of Helix Software's Multimedia Cloaking product. It uses Cloaking to relocate and run in protected mode on 386 processors.
The driver uses the Microsoft Networks interface in MS-DOS. This is the reason that at least version 3.1 of MS-DOS is required. The driver essentially looks like a network drive from the system perspective. It's implemented as a TSR program and an extension to the redirector interface (CDEX).