A waterproof polyurethane coating on the inside provides the waterproof barrier and the outer material is usually a pack cloth made of nylon – example 220 Denier or 420 Denier thread. These two materials are laminated together to form the bilaminate. The material itself is durable.

Dry bags are often used in kayaking, canoeing, rafting, canyoning, and other outdoor activities in which sensitive items would otherwise get wet, as well as extreme sports such as skiing and snowboarding.

The difference between waterproof and water resistant seems obvious. It's right in their names. Waterproof means water won't get through the bag. Water resistant means the bag will hold up against water but, at some point, water will get through.

To check, look at the bag claiming to have waterproof zippers. If the bag is sewn, it is very unlikely that the manufacturer used waterproof zippers. After all, why put a very expensive zipper on a less expensive water-resistant fabric?

Waterproof breathable fabrics consist of an outer layer called the “face fabric”, usually made of nylon or polyester, and a laminated membrane or coating, usually made of ePTFE (expanded Polytetrafluoroethylene, also known as Teflon) or PU (Polyurethane).

Dry Bags provide at Watertight enclosure by the way they are secured. ... Instead of a Ziploc type of closing mechanism, the bag is secured by rolling down or dog-ear the top at least 3 times, them clipping the buckles together. See my post about the proper way to close a dry bag here.