Hello Parth

Sheath is the outer covering a cell can develop that can be mucilaginous , glycoprotein, pectinase etc. The cell develops sheath according to it's own need for the purpose of cell enlargement, call protection etc.

Cell membranes are the outermost cell organ and act as a barrier and gate to control the transportation of materials and information between the cytoplasm and the external environment.

Hello dear candidate. 

A sheath is a layer. This layer can be composed of any organic or inorganic material. And a cell membrane is the outer covering of a cell, which encloses the protoplasm of the cell. It is a double layer structure, made up of lipids and proteins.