Sheer: It is a very thin transparent cloth which is basically used back of the curtains, for the purpose        

               of sunlight to  pass through when the curtains are tied back.

   Satin/Valance: This type of cloth is basically used to design the top of the curtains.

   Curtain Fabric: This is general fabric which is used to make the curtains.

                                  The Following picture depicts all the three.