Diffusion is the spreading out of molecules from an area of high concentration to an area of lower concentration of that type of molecule.  Diffusion is Nature's way of saying, "How about we go somewhere where it's not so crowded?"  Molecules are always in motion; they naturally move towards an area where there are fewer molecules.

   Passive transport is a term used to describe diffusion through a membrane.  The walls of the small intestine, of blood vessels and of cells themselves are permeable membranes which means they allow certain molecules to diffuse through them.  In this way, digested food molecules can be 'passively transported' from the intestine, into the blood vessels and eventually through cell membranes into the cells.

   Active transport is similar to passive transport, but in active transport the cell membranes use energy to "pull" molecules through the membrane, rather than just allowing the molecules to diffuse through the membrane.