IDML stands for InDesign Markup Language. IDML files are ZIP archives, containing numerous XML files. This set of files represent a complete InDesign document, including not only their design content, but also the settings and resources in effect when they were saved. Its primary uses are to provide some compatibility between documents created in different versions of InDesign, and to allow documents to be exported for manipulation by other tools.
IDML (InDesign Markup Language) is an XML-based format, introduced in Adobe InDesign CS4, for representing InDesign content. IDML is used in several InDesign and InCopy file types:
IDML files (or packages) are ZIP archives, containing numerous XML files. The entire set of these files represent a complete InDesign document. The files in an archive are described thoroughly in Adobe InDesign CS4 IDML File Format Specification. IDML files are identified by the *.idml file extension.