Ducts: In everyday life the term “Duct” is used for  any pipe, tube, or channel by which a fluid or other substance (like electric power line, telephone cables, or other conductors)  is conducted or conveyed.

In context of prestressing (particulary post tensioning)  this duct is normally formed by spiral-wound corrugated sheet metal, corrugated plastic tubing, metal pipe, or plastic pipe. This duct forms a conduit to accommodate post-tensioned element ( like strands etc) and provides an annular space for grout. This is called as “Prestressing duct”.

Sheathing: The term "Sheath" means a  case for a blade (or  knife). The word is used to mean any covering that is applied like or resembles in appearance or function the sheath of a blade.

"Sheathing" means the material used to sheathe (to case or cover).

In context of prestressing (particulary post tensioning) Sheathing is used as a general term for the duct material surrounding the prestressing element to provide corrosion protection or conduit for installation which also provides an annular space for grout. Sheathing is a material encasing prestressing steel to  prevent bonding of the prestressing steel with the surrounding concrete., provide corrosion protection & contain the corrosion-inhibiting coating.

	Transverse & Longitudinal ducts/sheathing

References:
1. Prestress Manual by Dept of Transportation, California
2. "What is Post Tensioning?" by  Evaluation and certification Service, LLC