ISO9660 (or ISO-9660 or even ISO 9660) is the a file system standard for CD-ROM encoding. The standard itself is pretty old, and shows DOS roots. In particular, file and directory names are limited to alpha characters, numbers, and underscores, and names are limited to the 8.3 format.

Before it was an ISO Standard, it was called High Sierra.

Sometimes you will find the string ";1" after all the file names. This is the version number, required by the standard. They are usually hidden from the user by the OS, but in the case of older HPUX (or maybe HP-UX?) systems, they can really be a pain -- the semi-colon (or maybe semicolon?) is usually used as an end-of-command marker. You must escape the filenames to use them correctly from most shells. Some burners have the option to omit these version numbers. The resulting disk will break the ISO spec, but people still using older HPUX will be thankful. A better solution is to upgrade the box... Those versions of HPUX also have y2k problems.

There are a number of extensions to the ISO spec that make it manageable by modern users. Microsoft's Joliet extension provides 64 character long file names and code page support for foreign character sets. The UNIX Rock Ridge extension allows longer files names, deeper nesting of directories, and also stores UNIX permissions. There is also apparently a Romeo version as well, which is similar to Joliet, but with 128 character long filenames. The El Torito Standard extensions provide booting from a CD.

The extension situation has gotten to the point now where it is almost worse than the early lack of a standard. Apparently, WinNT cannot read Romeo version disks, and it was only somewhat recently that Linux could handle Joliet disks. The pure ISO standard is supported almost everywhere, now, yet the dream of a true platform-independent medium is seemingly farther off now than it was before the ISO standard.