Passive Transport

     Passive Transport is any method of transporting materials that does not use energy.  Because of this, it is generally only found when particles move down their concentration gradient, from an area of higher concentration to an area of lower concentration.  One of the most common types of Passive Transport is Osmosis, which is solely responsible for moving water through a cell's membrane.  Other kinds of particles can also move through the cell membrane as well.  They are usually very small molecules such as oxygen and simply pass between the spaces in the lipid bilayer of a cell's membrane.  Passive Transport really isn't a very complicated process and as such is fairly easy to understand.  However, it is a vital process that all cells need to survive.  

See Also:  
-Osmosis
-Active Transport

Facilitated Diffusion; Image in Public Domain