HDF is a general purpose library and file format for storing
scientific data. HDF can store two primary objects: datasets and
groups. A dataset is essentially a multidimensional array of data
elements, and a group is a structure for organizing objects in an HDF
file. Using these two basic objects, one can create and store almost
any kind of scientific data structure, such as images, arrays of
vectors, and structured and unstructured grids. You can also mix and
match them in HDF files according to your needs.

This is the legacy version HDF4. Although it is still maintained, new
users that are not constrained to using HDF4, should use HDF5 instead.