Waterproof is about submersion
If a backpack is labeled waterproof, that means you can completely submerge the bag in a body of water without letting moisture inside. It goes beyond a rainstorm and optimizes for more extreme conditions.
You can walk through a waterfall or white water raft down a river with a waterproof backpack, and your stuff will stay dry.
To clarify, backpacks made with waterproof materials are not necessarily waterproof backpacks. A fabric can be waterproof. A zipper can be waterproof. And yet, a backpack made with waterproof fabrics and waterproof zippers may be classified as "water resistant" - not fully waterproof.
Creating a waterproof bag, one that keeps your stuff dry when you submerge the bag in water, is a huge endeavor.
Designers have narrow options and have to make sacrifices in the pursuit of waterproof. Every material need to pass a submersion test, and the components must be assembled in a way that doesn't create holes. If you sew two pieces of fabric together, you create a hole for water to seep through with every stitch. Those holes are too minuscule to impact a bag's performance in a rainstorm, but matter a lot when you submerge a bag in a lake.
Most companies approach waterproof assembly by welding thermoplastics - essentially melting two pieces of fabric together using ultrasonic sound waves.
Welding is an expensive way to make a backpack, and it requires very particular fabrics. Not every fabric can be welded. For that matter, many waterproof fabrics cannot be welded and therefore cannot be made into a truly waterproof bag. To top it off, waterproof bags also must be simpler designs than sewn bags due to the limitations of welding. That's why most are designed as a bucket-like main compartment with one or two flat pockets. Your choices in aesthetics and functionality are limited with a waterproof bag.
Waterproof backpacks are great at keeping water out when you're kayaking down a river, but the strict limitations in materials and assembly make them unideal for other circumstances.
Water-resistant can mean different things
So, what makes a backpack water-resistant vs waterproof?
The short answer: it depends. Unlike "waterproof," which has a clear definition for bags, water-resistant is a little vague.
A water-resistant backpack might only keep your stuff dry in a drizzle, or it might perform well in a downpour. To understand where a backpack falls in that spectrum, check for two main factors: zippers and fabric.
If you see coated zippers on a backpack, that's a clue your bag will perform well in heavy rain. Coated zippers keep water out of the bag in one of the most vulnerable places, since zippers are full of holes.
Fabric is a little more complicated. Some "waterproof" fabrics are simply tightly-woven nylons with a durable water resistant (DWR) coating applied to the back. No matter how tightly you weave a fabric, there will always be holes. Holes mean water gets through. Cheaper fabrics combat this issue with a coating to repel moisture. That coating works for a while... but it's infamous for rubbing away over time.
Higher quality waterproof fabrics have multiple layers to trap moisture and keep coatings from wearing off. The Outbreaker Collection has both a layer of waterproof PET film and a weather-resistant coating. It'll keep your stuff dry, even if you're stuck in a downpour.
As its name implies, sailcloth was originally used for the sails of racing boats and is made to stand up to extreme conditions. Sailcloth is an expensive, premium fabric choice for maximum performance and durability. If you see "sailcloth" or multiple waterproof layers in a backpack's fabric, that's a clue that your stuff will stay dry in heavier rain.
When in doubt, check with your backpack's manufacturer.
The Bottom Line
A high-quality, water-resistant backpack will keep your stuff safe and dry in a rainstorm. If you throw a water-resistant backpack into a lake, however, your laptop may not fare as well. If you're kayaking, bring a truly waterproof dry bag.