The OPeNDAP website defines itself as
OPeNDAP provides software which makes local data accessible to remote locations regardless of local storage format. OPeNDAP also provides tools for transforming existing applications into OPeNDAP clients (i.e., enabling them to remotely access OPeNDAP served data).
So you can basically think of it as a way to access remotely-stored datasets from your desktop. The key to doing this is the OPeNDAP client. Three sets of clients will be discussed in this document.
MEXCDF/MEXNC.
Clients provided by OPeNDAP itself.
NCO utilities.
You may have heard the term "DODS" previously. "OPeNDAP" is the new preferred term, but you can use the two interchangeably.