The new Office format will essentially be a zip file of modules containing everything relating to a particular document, spreadsheet or presentation. One module will contain the basic XML file and another will list document properties.
Others will hold document properties, custom XML definitions, embedded code and macros, comments, and charts. Presumably some of these modules will be optional, or empty, and Microsoft says they will al be accessible using standard unzipping tools..
There will be XML formats for Word, Excel and Powerpoint, distinguished by an ‘x’ added to the old three-letter suffix: docx; .xlsx, or pptx. Respective macro-enabled versions will be suffixed .docm, xlsm, and pptm.