Water resistant: Water resistant bags are made with fabrics that shed water because of their weave or because they have been treated. They are able to resist the penetration of water to some degree but not entirely. Since the bags use standard zippers with holes between the teeth and are stitched together using traditional sewing methods that leave puncture holes at the seams, they will soak through in heavy rain.
Water repellent: Water repellant bags offer a bit more protection than water resistant ones as they are made with fabrics that are more effective than water-resistant fabrics. The fabrics are either very tightly woven or coated with a finish that causes the water to repel by making little beads when it hits the fabric rather than going through. As a result, they are not easily penetrated by water. However, because water repellant bags also use standard zippers with holes between the teeth and are stitched together, leaving needle holes at the seams, there will be leakage in a serious downpour.
Waterproof: waterproof bags are also known as dry bags. They are impervious to water. Dry bags are typically made of PVC or Thermoplastic Polyurethane (TPU) coated fabrics. The plastic coating makes the fabric completely waterproof and can not be penetrated by water. The bags are then constructed with ultrasonic welded seams and are fitted with waterproof zippers. Because there are no needle or zipper holes, the bags are completely waterproof and not just water resistant. Moreover, the roll-top closure, where the opening folds over on itself two or more times, keeps the bags air and watertight.
If there are stitched holes where the body panels meet, it isn’t waterproof!