An air barrier stops air from moving in or out of the conditioned space, effectively blocking the air pressure differences that drive the stack and wind effects. Air barriers can be in place anywhere in the building envelope, and there can be more than one air barrier. Air barriers can be created from a wide range of materials: polyethylene sheeting, house wrap, self-adhered membranes, boardstock, board insulation, spray polyurethane foam, poured concrete, metal, glass, and a host of other materials.
A vapour barrier stops water vapour from diffusing through materials -- and so there is a more descriptive name: vapour diffusion retarder. In cold climates like Canada’s, the vapour barrier should be on the inside of the insulation. In hot climates, it should be installed on the outside of the insulation. In both cases, the vapour barrier prevents warm, humid air from leaving its moisture behind as it meets a cool surface, regardless of the direction it is moving.
Things get complicated in energy efficient construction, because a material can be an air barrier, a vapour barrier, a weather barrier, an insulator or any combination of those four functions.