The Mac OS plays well with others, recognizing that there are, in fact, other file systems in the world. As such, you can mount removable media that were intended for other operating systems without much fuss.
Some removable media, however, are hybrids: they may contain a Mac OS directory structure as well as a Windows directory structure. (As far as I know, only Macs can create these hybrid CDs, using software like Toast.) Many games that are packaged for both Windows and Mac OS are delivered on hybrid CDs. When you insert the CD into your drive, the OS looks at the CD, finds the Mac portion, and mounts it as a native Mac volume.
So what if you have a hybrid CD, and you want to mount the non-native volume? Hold down Command-Option-i while inserting the disk, and this will force the Mac OS to mount the ISO 9660 volume.