ISO 15924, Codes for the representation of names of scripts, defines two sets of codes for a number of writing systems (scripts). Each script is given both a four-letter code and a numeric one. Script is defined as "set of graphic characters used for the written form of one or more languages".
Where possible the codes are derived from ISO 639-2 where the name of a script and the name of a language using the script are identical (example: Gujarātī ISO 639 guj, ISO 15924 Gujr). Preference is given to the 639-2 Bibliographical codes, which is different from the otherwise often favored use of the Terminological codes.
4-letter ISO 15924 codes are incorporated into the Language Subtag Registry for IETF language tags and so can be used in file formats that make use of such language tags. For example, they can be used in HTML and XML to help Web browsers determine which typeface to use for foreign text. This way one could differentiate, for example, between Serbian written in the Cyrillic (sr-Cyrl) or Latin (sr-Latn) script, or mark romanized text as such.
More information on the Wikipedia page .