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What is a membrane?

Membrane is any delicate sheet that separates one region from other blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered a membrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as membrane too.

Concerning their permeability how are membranes classified?

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

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

What is diffusion?

Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to other region where it is less concentrated. For example, during the boiling of water in a kitchen gaseous water particles tend to uniformly spread in the air by diffusion.