Hydraulic fittings are designed to connect hydraulic hoses, tubes and pipes together in hydraulics, these fittings enable you to contain, direct and control the flow of hydraulic pressure. Such as compression fittings, crimp fittings, flange fittings, push to connect fittings.
Hydraulic couplings are a fitting used to connect pieces of hose, pipe or tube, usually, these are used with hydraulic hoses, they are often made from steel as it can withstand the pressure and is corrosion resistant which is important as hydraulics aren't only water-based some hydraulic fluids are acid or oil based.
Couplings are when both ends to be attached to the hose, pipe or tube, you can also get reducer couplings that are smaller on one end, and adaptors have separate endings, for example, one BSP male to female.