Passive transport is a type of cellular transport where substances move along the concentration gradient. Since it is associated with downhill movement of substances (i.e. from an area of higher concentration to an area of lower concentration) it does not require a chemical energy. It is driven though by kinetic and natural energy. This is in contrast to an active transport, which is involved in uphill movement of substances (i.e. from lower to higher) and therefore requires a chemical energy, e.g. ATP.
There are four major types of passive transport are (1) simple diffusion, (2) facilitated diffusion, (3) filtration, and (4) osmosis. Simple and facilitated diffusions refer to the net movement of molecules from higher to lower concentrations. Osmosis refers to the diffusion of a solvent (usually water molecules) through a semipermeable membrane from lower to higher solute concentrations. Filtration is the movement of water and solute molecules across the cell membrane driven by hydrostatic pressure that is generated by the cardiovascular system.