A vapor barrier in the building industry is typically polyethylene sheeting, or better known as plastic.  To qualify as a vapor barrier, the poly must be a minimum of 4 millimeters thick which is the most commonly used thickness in residential construction.  A vapor barrier placed under a concrete floor is typically 6 millimeters thick to help prevent punctures during concrete placement.  The objective of a vapor barrier placed on the heated side of a wall is to stop the transfer of water vapor from entering the wall cavity.  Vapor entering walls cavities will find its condensation point within the wall during the winter where it becomes liquid and can cause a multitude of problems.  A vapor barrier placed under a concrete floor stops water, both liquid and vapor, from increasing the moisture content of the floor.