The Apache POI Project's mission is to create and maintain Java APIs for manipulating various file formats based upon the Office Open XML standards (OOXML) and Microsoft's OLE 2 Compound Document format (OLE2). In short, you can read and write MS Excel, MS Word and MS PowerPoint files using Java. The Apache POI project has a complete API for porting other OOXML and OLE2 formats.
OLE2 files include most Microsoft Office files such as XLS, DOC, and PPT as well as MFC serialization API based file formats. The project provides APIs for the OLE2 Filesystem (POIFS) and OLE2 Document Properties (HPSF).
Office OpenXML Format is the new standards based XML file format found in Microsoft Office 2007 and 2008. This includes XLSX, DOCX and PPTX. The Apache POI project provides a low level API to support the Open Packaging Conventions using openxml4j.
For each MS Office application there exists a component module that attempts to provide a common high level Java api to both OLE2 and OOXML document formats. This is most developed for Excel workbooks (SS=HSSF+XSSF). Work is progressing for Word documents (HWPF+XWPF) and PowerPoint presentations (HSLF+XSLF).
The Apache POI project has recently added support for Outlook (HSMF). Microsoft opened the specifications to this format in October 2007. We would welcome contributions.
There are also projects for Visio (HDGF), TNEF (HMEF), and Publisher (HPBF).
As a general policy the Apache POI project collaborates as much as possible with other projects to provide this functionality. Examples include: Cocoon for which there are serializers for HSSF; Open Office.org with whom the project collaborates in documenting the XLS format; and Tika / Lucene, for which we provide format interpretors. When practical, Apache POI donates components directly to those projects for POI-enabling them.