This is a quick overview of the RainScreen concept.
In a rainscreen design the exterior surface of the wall is not considered a weather barrier as in most traditional wall systems. Think of wood siding; nothing is intended to get past the siding. In a rainscreen wall, most water is kept out but not necessarily all water. Any water that makes it through the screen is conducted down through the air space and back to the exterior. The archetypal example is a masonry cavity wall. Rather than just one line of defense, the rainscreen provides two lines of defense and the critical second line of defense is subject to only a fraction of the rain water, and it is not wind-driven by the time it reaches the inner barrier.
A rainscreen wall is becoming the expected, modern solution to exterior walls. The benefits outweigh the disadvantages. Where budgets allow and capable workmen can be counted on to implement the design, the rainscreen is the best solution. Bear in mind that it will take more design effort to create a good rainscreen, and nothing can be readily delegated to the contractor, unless he is a specialty rainscreen contractor with proven experience (there are none in many areas). A good design will take more research, detailing, and follow-thru during construction than a barrier wall. Because of this attention to detail, not every project is a candidate for a rainscreen.
The diagrams below were taken from Wikipedia - http://en.wikipedia.org/wiki/Rainscreen_cladding