Rain shield or rain screen is a term used to describe an exterior cladding that has an air space behind it. In contemporary construction it's done to prevent trapping rain or condensation between the cladding and the exterior sheathing or wind barrier. It can act as a radiant barrier space and also a thermal gap to prevent conduction from the framing to the exterior. The draw back is that it gives bugs a great hiding place and makes a wall assembly thicker for not a lot of gain in true insulation performance.
The weight of any interior wallboard is good to keep in mind. A sill plate could be bolted to the foundation and used to support the weight of the wall off the framing. Use good pressure treated lumber and it should last plenty long enough. As long as you're not framing a lumber wall to hold the interior sheathing, the slab should be sufficient to hold up the dead load it may impart. Metal siding would likely be the lightest per square foot and drywall (1/2 or thicker) the heaviest. You can look at lightweight drywall too, which is easier to handle if nothing else.
