Moisture vapour will move by diffusion from an area of high concentration (pressure) to one of lower concentration. Unimpeded, this happens readily. However, barriers such as fabrics reduce the rate at which this occurs. It is crucial to remember that vapour pressure drives moisture vapour: humidity is not the essential factor.
Modern waterproof-breathable fabrics can be divided into three different types, and each work in slightly different ways. It is not entirely accurate to state that fabrics can be both breathable and waterproof because the holes in their structures are large enough to let water vapour through, but too small to let water droplets through.
Fabrics with microporous coatings or membranes are prevalent in many garments. PTFE (polytetrafluoroethylene) and PU (polyurethane) tend to be used to make these coatings and membranes. They act as a filter: their microporous structures contain billions of holes that link together in contorted pathways. They stop water penetrating them by maintaining a very low Surface Energy. However, if the membrane or coating becomes contaminated then they can leak, as their Surface Energy increases. Microporous membranes are usually made by mechanical fibrillation, whereas microporous PU coatings are usually produced by wet coagulation.
eVent (BHA Industries) is a well-regarded microporous PFTE membrane. Its structure is protected from contamination by lining the pores with hydrophobic and oleophobic chemicals. By doing this, eVent remains air permeable, which maximises its ability to transmit water vapour.
Fabrics with continuous Hydrophilic? coatings or membranes contain no pores, making them impermeable to air. They are usually made of PU and PEO (polyethylene oxide). Moisture vapour transport in these structures occurs by molecular wicking: the water molecules are first adsorbed to the surface of the Hydrophilic? material, then they desorb and adsorb to the next molecule along. This process continues throughout the thickness of the Hydrophilic?.
The breathability of Hydrophilic? materials tends to be slightly lower in lab tests than that of PTFE membranes. However, their breathability is strongly affected by temperature and they are developed to operate best at temperatures just above freezing, whereas many lab tests are conducted at skin temperature.
Modern Gore-tex? is a bicomponent microporous and Hydrophilic? laminates: holes of the microporous PTFE membrane are partly-filled with Hydrophilic? polyurethane. This leads to excellent durability, though the structure is impermeable to air, limiting the maximum possible breathability. Water vapour is transmitted through the structure in a way analogous to that of the Hydrophilic? fabrics.
General Links for Basic principles:
Waterproof Breathable Active Sports Wear Fabrics: This document expands upon the information given here, explaining moisture transport and how this is achieved in textiles.
Video outlines how the eVent fabric technology works.