Q. Concerning their permeability how are membranes classified?

Membranes can be classified as permeable, impermeable, selectively permeable or semipermeable.

An impermeable membrane is so that through which no substance can pass. Semipermeable membranes are those that let barely solvents, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like molecules and ions, to pass across it. There are as well selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent let only some specific solutes to pass while blocking others.