Waterproof hiking gear is pretty much what it sounds like, but there's a catch: Not all waterproofing is equal. Some garments can protect you in a torrential deluge, but others will eventually let the water come through.
Also, if your garment is waterproofed with an exterior coating -- DWR for example -- that coating can abrade away with normal use, leaving patches of your gear (and you beneath) with substandard protection. * The good news is that waterproof coatings like DWR can be renewed with wash-in or spray-on treatments.
No matter how your garment is waterproofed, odds are good that it'll have a specific waterproof rating attached to it (check the tag; not all manufacturers include this information, but many do).
More good news: Unlike breathability, there is an industry-wide standard for measuring waterproofness. Mind you, there's no single definition for just how much water a garment should withstand before it earns the label "waterproof" -- but as long as the manufacturer discloses its products' waterproof rating on the hang tag, you can decide for yourself whether you trust it to be truly waterproof in the field.
Waterproofing for outdoor gear is usually reported in millimeters (mm) or psi (pounds per square inch). To convert from mm into psi, divide by 704; to convert from psi to mm, multiply by 704. The actual testing is usually conducted with a water column test, although you might occasionally see results from a rain room test as well.The Water Column Test
An open-bottomed tube is placed over the fabric and gradually filled with water (creating the water column, natch). The rating generated by this test is how tall that water column got -- which can be converted to how much pressure it exerted if you divide by 704 to go from mm to psi -- before the fabric started to leak.The Rain Room Test
The fabric is placed in a "weather room" and pelted with simulated rainfall. For the sake of reference, REI's guide to rainwear says that wind-driven rain usually measures about 2 psi of force, and hurricane-driven rain measures about 10 psi.
Chinks in Your Armor
No matter how waterproof a given fabric is, you can still end up all wet through user error or poor garment construction. The most common mistake I've seen is raising an arm over your head with your pit zips open; it creates a perfect waterslide to transport any precipitation right through that open armpit zipper. If your jacket or pants don't fit well, you might also create gaps for the rain or water to flow through when you raise your arms, bend over, or sit down.
Seams are another weak point: the best waterproof gear will have seams that are taped, welded or otherwise sealed to cover tiny holes left from the stitching. You can also choose to seal the seams yourself (a common practice with new tents).
Zippers should either be welded for the same reason or, better yet, have a storm flap that covers them to keep water from coming in. And finally, be aware that when you sit on a wet surface in your nice waterproof pants, you're exerting significant pressure -- maybe enough to let the water start seeping through.
Waterproof footwear is a double-edged sword; although it's a must for protecting your feet on long slogs through wet areas, the waterproofing only protects you up to the top of the boot or shoe -- and if water makes it over the top (or if your feet sweat enough to wet you down), that waterproof membrane or coating is going to keep the water in.
I love my waterproofing hiking boots (can you guess from the picture?) but on hikes where I know I'm going to have to wade through a significant stretch of water or mud, I'll often hike in my trusty Xtratuf rubber boots -- the kind you see fishermen wear. They're taller and more waterproof -- impermeable, really -- than any hiking boots I own, and although they're not the epitome of comfort, they're not too bad if you wear thick, tall socks for padding and to keep the clammy rubber away from your skin.
Most backpacks and tent bodies are water-resistant at best. That's why you have a waterproof rainfly to pitch over your tent, and some backpacks come with a built-in raincover that you can pull out and slip over the pack to keep water from seeping in through the zippers or fabric.
If your pack doesn't have a built-in raincover, you can usually purchase one separately or create your own from a garbage bag or, if you're feeling fancy, you can use sil-nylon or other waterproof fabric.
*Make sure you follow the manufacturer's directions for laundering your outdoor gear so you prolong, instead of shortening, its waterproofing ability.