A non-porous material is one that cannot absorb liquid or gas. Glass, metals and hard plastics are examples of non-porous materials.
Porous materials have holes or pores which makes it easy for gas or liquids to pass through them. A sponge is an excellent example of a porous material. Semi-porous materials allow certain liquids or gas through, usually by diffusion. An example of semi-porous materials are those used to purify water. Human skin is is also a semi-porous material.
A non-porous material lets nothing through. It is generally too hard for things to pass through and doesn't have holes to allow substances in or out.