(Source: HDF-EOS FAQ from https://wiki.earthdata.nasa.gov/display/DAS/HDF-EOS+FAQ)
The HDF-EOS data format is standard HDF with ECS conventions, data types, and metadata added. HDF-EOS adds three geolocation data types (point, grid, and swath) which allow the file contents to be queried by earth coordinates and time. An HDF-EOS file also contains ECS core metadata which is essential for ECS search services. An HDF-EOS file can be read by any tool that processes standard HDF files, although HDF-EOS geolocation or temporal information generally is not accessible using standard HDF calls.
A data product need not fit any of the grid, point or swath models to be considered HDF-EOS. If ECS metadata are included, it is a valid HDF-EOS file.
HDF-EOS is implemented as a C library extension of the standard HDF library (with FORTRAN bindings). HDF-EOS ordinarily is based on the latest release of HDF.