HDF5 is a completely new Hierarchical Data Format product consisting of a
data format specification and a supporting library implementation. HDF5 is
designed to address some of the limitations of the older HDF product and to
address current and anticipated requirements of modern systems and
applications.

HDF5 includes the following improvements.
   - A new file format designed to address some of the deficiencies of HDF4.x,
     particularly the need to store larger files and more objects per file.
   - A simpler, more comprehensive data model that includes only two basic
     structures: a multidimensional array of record structures, and a grouping
     structure.
   - A simpler, better-engineered library and API, with improved support for
     parallel I/O, threads, and other requirements imposed by modern systems
     and applications.

WWW: http://hdf.ncsa.uiuc.edu/HDF5/