The format of an HDF5 file on disk encompasses several key ideas of the HDF4 and AIO file formats as well as addressing some shortcomings therein. The new format is more self-describing than the HDF4 format and is more uniformly applied to data objects in the file.
An HDF5 file appears to the user as a directed graph. The nodes of this graph are the higher-level HDF5 objects that are exposed by the HDF5 APIs: Groups, Datasets, Committed (formerly Named) datatypes.
At the lowest level, as information is actually written to the disk, an HDF5 file is made up of the following objects: A superblock, B-tree nodes, Heap blocks, Object headers, Object data, Free space