HyVent is a waterproof and breathable shell fabric produced by The North Face. It's basically a polyurethane coating (called a Durable Water Resistant or DWR coating) on top of a synthetic fabric that only allows moisture to move one way. This keeps water out of the garment while allowing water vapour (evaporated sweat) to move through the jacket. The major benefit to this is that it stops you getting sweaty! This may not seem that amazing, but if you've ever spent a whole day walking or climbing, you'll sweat around 0.25 to 0.5 litres an hour. So in an 8 hour day, you could sweat up to 4 litres; if your jacket doesn't let that evaporate off, imagine pouring 8 pints of water over yourself!
More effective waterproof and breathable fabrics (such as Gore-Tex or eVent) use a membrane in 2 or 3 layers to achieve this, but this makes the fabric much more expensive, so HyVent is a good compromise of functionality versus price. However, HyVent is neither as waterproof or as breathable as fabrics with a membrane.
Due to the DWR treatment on the outside of the fabric, it will rub off over time, so it's necessary to treat HyVent regularly with a treatment to repair the coating. Nikwax produce a number of these such as Techwash or TXDirect. Washing with detergent will also remove the DWR coating, so never put your waterproof in the washing machine! You can wash it on a cold wash with some Techwash or TXDirect wash-in, but never put soap powder in.
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