Water-borne adventures can be some of the greatest you'll have. The river goes where the roads can't, creatures of the water fascinates those of us, and you get to mess around in boats. Which is great until you discover that your gear is soaked. Waterproof bags are easy to find, inexpensive, and very durable.
The key factor that defines a piece of gear or clothing as waterproof is the seam construction. Any sewn seam has stitch holes that will let water in unless sealed. At one time, most outdoor gear required some owner maintenance and application of silicone seam seal, but with the refinement of fabric 'welding' and bonding technology, that now happens at the factory. Your tent, jacket, or bag will have what looks like a piece of tape running along the seam, assuring you that water isn't coming in. If the seams aren't sealed, it's not waterproof, no matter what fabric the item is made of.
The majority of dry/waterproof bags have very simple construction and use a roll-top opening to achieve a watertight closure. To seal them, the top is rolled down and a quick-release buckle is clipped to hold the roll. This basic opening will be found in stuff-sack, backpack, and duffel designs. Some duffel-style dry bags may have true dry zippers, featuring large teeth with a rubber gasket that is compressed by the zipping process.
Roll-top only designs are not considered submersible, but some dry zip and 'freezer bag' style closures can be completely submerged. Check the manufacturer's specifications for watertight or submersion ratings.
Backpack straps and side-opening duffel designs can make carrying and accessing your gear much easier, but they will add seams and complexity, possibly shortening the service life of your bag.
Dry bags are built of nylon-reinforced vinyl and all seams are welded, meaning the material is heated to bonding temperature along the seams, forming a very strong water- and airtight seal. The material and seams tend to be stiffer and less flexible than those with stitched seams. Vinyl has the real advantage that the rubbery outer shell material will not absorb any water, so the bag itself will not gain weight and dries (and cleans up) much faster.
Waterproof stuff sacks or bags will be made of durable waterproof-coated nylon with stitched and sealed seams very much like what is found on a rain jacket. They are fairly conventional designs and tend to be light, flexible, and compressible. The fabric may have a white interior coating on a conventional nylon pack fabric, or be silicone-impregnated silnylon, with very high water repellency as well as waterproofing that can be pinholed and reseal. Both sides of silnylon are relatively slick and it has a distinctive crinkle sound when handled. The fabric is very strong under tension but has low resistance to abrasion.
To compare the fabrics, a 30L SealLine reinforced vinyl Baja Dry Bag weighs 20 ounces, the Sea to Summit 35L coated nylon Dry Sack is 7.7 ounces, and a 35L silnylon Dry Sack is 2.2 ounces.
Paddlers have known about dry bags for decades now. The simplicity and durability mean throwing them around is easy, and in a pinch they can be used as buckets. For most other activities, the weight and bulk of a dry bag is prohibitive, so most backpackers and travelers opt for fabric waterproof stuff sacks to protect clothing and gear inside their packs.
For waterproof carrying over distance, the 'portage pack' with roll-top closure, welded vinyl fabric and rudimentary shoulder straps is better than hand-carrying a big single-handle cylindrical dry bag, but won't serve the commuter or hiker very well. As far as a truly waterproof backpack, few brands have the technology and willingness to completely seam-seal a good conventional pack design, and the time and skill required mean prices twice or three times higher than a 'regular' pack. With internal protection in the form of waterproof stuff sacks or a pack liner combined with a pack cover, your pack and gear will stay dry through most backcountry adventures.