ISO 9660 is the file system standard used on most of the CD-ROM and DVD-ROM discs. It is the most widely supported format for storing data on an optical discs.
The standard has three levels for storing the data. In Level 1 the length of the file names is constrained to the 8.3 format. Ie. a filename can have a 8 character body and a 3 character extension. The allowed characters for Level 1 are upper case letters, numbers, and the underscore. Level 2 allows file names of up to 31 characters. Level 3 is seldom used, and allows fragmentation of the files.
Due to the limitations of the ISO 9660 specification various extensions have been adopted to the standard. Rock Ridge preserves the Unix permissions and allows longer filenames. Joliet extends the character set to Unicode, which allows virtually any character to be used in a filename. It also allows longer filenames to be used.
Disc image files written in the ISO 9660 format are generally referred to as ISOs and have the ".iso" extension.
A much more versatile Universal Disk Format can be considered to be the successor of ISO 9660.