The bag materials that are genuinely waterproof are the ones with a continuous plastic layer that water cannot pass through. In bags, that means three main families.
PVC tarpaulin is polyester fabric coated on both sides with PVC, usually around 500D and 0.5 mm thick. It is tough, easy to clean, easy to print and the most cost-effective waterproof bag fabric, which is why it is used for most dry bags, boat bags and motorcycle bags. It is heavier than the alternatives and stiffens in the cold.
TPU-laminated fabric is nylon or polyester with a TPU film bonded to it, commonly in 210D, 420D or 840D. It is lighter and more flexible than PVC, stays flexible in cold weather and does not need plasticisers, so it is the usual choice for lightweight dry bags, fly fishing packs and higher-priced travel ranges.
Coated nylon and polyester with a PU or PVC coating can also be waterproof, but thin coatings wear through at rub points, so they suit linings and lighter bags better than heavy-duty use.
Fabric is only half the answer. A bag made from waterproof fabric still leaks if its seams are stitched, because every needle hole lets water through, or if it closes with an ordinary zip. Truly waterproof bags join the fabric with high-frequency welded seams and close with a roll-top, a waterproof zip or an airtight zip. For a deeper comparison, see our TPU waterproof material guide and our guide to TPU vs PVC dry bags.