A vapor barrier is a layer with very low water-vapor permeance. The broader term vapor retarder covers materials that slow vapor diffusion to different degrees.
What this means at your property
Vapor control is different from managing rain leakage or air movement through gaps. Adding an impermeable layer without considering the rest of a wall or floor can change where the assembly is able to dry.
Where should a vapor-control layer go?
The appropriate design depends on climate, assembly and use. Ask the designer or contractor to explain the moisture paths and product choice for the actual building, rather than applying a universal inside-or-outside rule.
Related terms
Further reading: Department of Energy: vapor barriers.