HDF4 (originally known as HDF) is file format for storing scientific data
and a software library that provides high-level APIs and a low-level data
access interface.

HDF technologies at present include two data management formats (HDF4 and
HDF5) and libraries, a modular data browser/editor, associated tools and
utilities, and a conversion library.  Both HDF4 and HDF5 were designed to
be a general scientific format, adaptable to virtually any scientific or
engineering application, and also have been used successfully in non-
technical areas.  The Open Source format is a key technological foundation
for HDF core technologies.  It allows users to collaborate with The HDF
Group regarding functionality requirements and permits users' experience
and knowledge to be incorporated into the HDF product when appropriate.

It also permits users and organizations without adequate technology
resources to use a sophisticated and robust data management tool for no
charge.

WWW: http://www.hdfgroup.org/products/hdf4/