What the heck is a Vapor Barrier?

Simply put, they prevent water vapor (gas) from traveling in a particular direction, or in some cases, in more than one direction.  The best discussion on Vapor Barriers that I know of is in an article by Building Science.com which has a wealth of information on many subjects relating to- guess what?  Closed-Cell spray foam is the best vapor barrier for building envelope applications becasue it is the only insulation material that is also a100% functional vapor barrier in both winter (heating season) and summer (air conditioning season).