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.

Why HDF5? The development of HDF5 is motivated by a
number of limitations in the older HDF format and library.

HDF5 includes the following improvements over HDF4

* 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.