An ion-exchange membrane is a semi-permeable membrane that transports certain dissolved ions, while blocking other ions or neutral molecules.  
Ion-exchange membranes are therefore electrically conductive.  They are often used in desalination and chemical recovery applications, moving ions from one solution to another with little passage of water.
Important examples of ion-exchange membranes include the proton-exchange membranes, that transport hydrogen| cations, and the anion exchange membranes used in certain alkaline fuel cells to transport Hydroxyl| anions.