This NHD watershed tool support-layers directory is distributed as an ARC/INFO 
tarred and compressed workspace.  This format is referred to as NHDinARC.  The
support-layers directory works dependent on the prior access of the Hi-Res NHD
dataset for the same 8-digit Hydrologic Unit (also known as a cataloging unit).
The workspace contains the data used to delineate watersheds from any point on
the NHD for the selected 8-digit Hydrologic Unit.  Within the workspace, the NHD
watershed tool support-layers are implemented in two ARC/INFO Grids, one coverage,
a shapefile (in the dedrite folder) and several related INFO tables.  There is 
also a folder that contains a series of text files for the metadata.  Now that you 
have uncompressed and untarred the workspace, you should see the following folders.
To understand more about these datasets and how they were developed, see the metadata.

 01080105_ws (just an example CU number)
    - info
    - metadata (a workspace containing metadata)
    - dir_grd (an arcinfo grid layer)
    - accum (an arcinfo grid layer)
    - shed_cov (an arcinfo polygon coverage)
    - dendrite (a workspace containing shapefiles)