HSLF is the POI Project's pure Java implementation of the Powerpoint '97(-2007) file format.
HSLF provides a way to read, create or modify PowerPoint presentations. In particular, it provides:
The quick guide documentation provides information on using this API. Comments and fixes gratefully accepted on the POI dev mailing lists.
XSLF is the POI Project's pure Java implementation of the PowerPoint 2007 OOXML (.xlsx) file format. Whilst HSLF and XSLF provide similar features, there is not a common interface across the two of them at this time.
Please note that XSLF is still in early development and is a subject to incompatible changes in future.
A quick guide is available in the XSLF Cookbook