What makes an outdoor jacket actually waterproof versus water-resistant?

A water-resistant jacket only repels light rain; a waterproof jacket keeps water out under sustained pressure. Water resistance comes from a DWR (durable water repellent) finish on the outer face — droplets bead up and roll off, but the fabric will soak through in steady rain.

A waterproof jacket adds a membrane (e.g. PTFE or TPU film) or coating bonded to the fabric, plus taped or sealed seams. Performance is measured as hydrostatic head in millimetres (ISO 811 test): 5,000 mm suits drizzle, 10,000–15,000 mm covers hiking and commuting, and 20,000 mm handles sustained alpine rain. When the DWR wears off, water can wet out the face fabric — but a true waterproof membrane still blocks penetration.