Walking my dog often immerses me in weather I would otherwise avoid. I've learned that the appropriate outwear can make inclement weather a lot more bearable. While looking for a new rain jacket, salespeople kept telling me about the importance of finding something waterproof that breathes (WP/BR). After looking through the many different kinds, I'll only talk about laminates, such as Gore-Tex (the native apparel of Vancouverites) because they are most mysterious.
Breathability
When they (the people selling these things) say breathable, they do not mean you could live underwater if you wear these garments. They mean that your perspiration can escape as water vapour so you don't get soggy and cold. Testing for breathability is not standardized, but one method is to seal a quantity of water with the material and see how many grams of water per square meter evaporates in 24 hours. Good breathability starts around 5000. But temperature and humidity can affect these rates differently for different materials.
Waterproofness
Some materials are completely waterproof, but they are not breathable. Measuring the waterproofness of breathable materials is also not standardized, but one way includes how many millimeters of water can stand on the material without seeping through for 24 hours. "Good" starts around 10000, though this is often just tested on the ePTFE part and not the outer fabric.
ePTFE
The special thing about laminates is the expanded polytetrafluoroethylene (ePTFE). Bob Gore invented ePTFE in the late 60s for the Gore family business which deals with all things PTFE, also known as Teflon, of non-stick cookware fame. The ads always talk about how ePTFE has 1.4 billion pores per square centimetre and that the rain stays out because these pores are 20000 times smaller than a drop of liquid water. But perhaps more importantly, ePTFE has low surface energy so the water molecules prefer to stick to each other than to it (it's hydrophobic). They also say the pores are 700 times bigger than molecules of water vapour, so your perspiration can pass out. But for water to penetrate ePTFE, a great force is required or something has to change the low surface energy.
PU
Dirt and oils can change this surface energy and make the ePTFE vulnerable to water, which they found out after the Gore-Tex laminate was first introduced back in 1978. Nowadays, Gore-Tex has an inner layer of a thin Polyurethane (PU) film for protection. The PU does not have pores but is hydrophilic, so water molecules can pass through it by diffusion. This works more slowly than ePTFE alone and assumes that inside (closer to you) has a higher concentration of moisture compared to the outside, which might not be true if it's actually raining. Instead of a PU film, a competitor called eVent, adds a secret coating to the ePTFE that protects it without clogging the pores. Usually the jacket itself has some other kind of material liner to keep the shell away from your grossness.
DWR
The outer surface is a more durable and potentially colourful material, like polyester or nylon, treated with some kind of Durable Water Repellant (DWR). The DWR causes water to bead and roll off instead of spread out so water is less likely to penetrate the jacket, while also still allowing for breathability. DWR contains fluorochemicals or silicone. Fluorochemicals have greater repellence but negative environmental impacts.
To remain effective, the DWR requires regular and proper cleaning. If the DWR is not very good or worn out, the outer material could be soaked with water without passing through the ePTFE layer below. The breathable systems work best when the environment is more dry, which might seem to defeat the purpose. And you might not notice the difference for what you do anyway. Which is why I decided not to fork out the big bucks on something just to walk my dog in the rain at a leisurely pace. So far it has just been so much water off a duck's back.