Packages 
Package Description
org.apache.poi  
org.apache.poi.common  
org.apache.poi.common.usermodel  
org.apache.poi.common.usermodel.fonts  
org.apache.poi.ddf
This package contains classes for decoding the Microsoft Office Drawing format otherwise known as escher henceforth known in POI as the Dreadful Drawing Format.
org.apache.poi.extractor  
org.apache.poi.hpsf
Processes streams in the Horrible Property Set Format (HPSF) in POI filesystems.
org.apache.poi.hpsf.extractor  
org.apache.poi.hpsf.wellknown
Specific support for DocumentSummaryInformation, SummaryInformation types.
org.apache.poi.hssf
Horrible SpreadSheet Format API's for reading/writting Excel files using pure Java.
org.apache.poi.hssf.eventmodel  
org.apache.poi.hssf.eventusermodel
HSSF eventmodel Package provides an event-based API for reading HSSF files.
org.apache.poi.hssf.eventusermodel.dummyrecord  
org.apache.poi.hssf.extractor  
org.apache.poi.hssf.model
Provides low level API structures for reading, writing, modifying XLS files.
org.apache.poi.hssf.record
Record package contains class representations for XLS binary strutures.
org.apache.poi.hssf.record.aggregates
The record aggregates are not real "records" but collections of records that act as a single record.
org.apache.poi.hssf.record.cf  
org.apache.poi.hssf.record.chart  
org.apache.poi.hssf.record.common  
org.apache.poi.hssf.record.cont  
org.apache.poi.hssf.record.crypto  
org.apache.poi.hssf.record.pivottable  
org.apache.poi.hssf.usermodel
The usermodel package maps HSSF low level structures to familiar workbook/sheet model
org.apache.poi.hssf.usermodel.helpers  
org.apache.poi.hssf.util
The util package contains tools needed for writing HSSF files that are not necesarily "real" HSSF concepts.
org.apache.poi.poifs.common
common package contains constants and other classes shared across all POIFS subpackages
org.apache.poi.poifs.crypt
Implementation of the ECMA-376 and MS-propritary document encryptions
org.apache.poi.poifs.crypt.agile  
org.apache.poi.poifs.crypt.binaryrc4  
org.apache.poi.poifs.crypt.cryptoapi  
org.apache.poi.poifs.crypt.standard  
org.apache.poi.poifs.crypt.xor  
org.apache.poi.poifs.dev
DEV package serves two purposes.
org.apache.poi.poifs.eventfilesystem
The eventfilesystem is an efficient method for reading OLE 2 CDF files.
org.apache.poi.poifs.filesystem
filesystem package maps OLE 2 Compound document files to a more familiar filesystem interface.
org.apache.poi.poifs.macros  
org.apache.poi.poifs.nio  
org.apache.poi.poifs.property
property package contains high and low level Property structures for POIFS.
org.apache.poi.poifs.storage
storage package contains low level binary structures for POIFS's implementation of the OLE 2 Compound Document Format.
org.apache.poi.sl.draw  
org.apache.poi.sl.draw.geom  
org.apache.poi.sl.extractor  
org.apache.poi.sl.image  
org.apache.poi.sl.usermodel  
org.apache.poi.ss  
org.apache.poi.ss.extractor  
org.apache.poi.ss.format
This package contains classes that implement cell formatting
org.apache.poi.ss.formula
This package contains common internal POI code for manipulating formulas.
org.apache.poi.ss.formula.atp  
org.apache.poi.ss.formula.constant  
org.apache.poi.ss.formula.eval  
org.apache.poi.ss.formula.eval.forked  
org.apache.poi.ss.formula.function  
org.apache.poi.ss.formula.functions  
org.apache.poi.ss.formula.ptg
The formula package contains binary PTG structures used in Formulas
org.apache.poi.ss.formula.udf  
org.apache.poi.ss.usermodel  
org.apache.poi.ss.usermodel.helpers  
org.apache.poi.ss.util  
org.apache.poi.ss.util.cellwalk  
org.apache.poi.util
Top-level util package are classes that are useful throughout the project.
org.apache.poi.wp.usermodel