Both External Frame (EF) and Internal Frame (IF) packs are typically made of one of four types/styles of fabric: Cotton Canvas, "Pack Cloth" Nylon, Cordura Nylon and Rip-stop Nylon

Cotton Canvas
Here's a fabric that's remained relatively unchanged for close to 100 years. Canvas is simply a heavy-weight, rough weave cotton material. Packs made from this material are usually NOT waterproof. About the only place in Canada or the U.S. you're likely to find packs made of this material are from the Boy Scouts or possibly the military. You might also find a number of inexpensive Oriental packs made from this material. In Europe, with a little searching, you can find some specialized packs made from canvas. For instance, one Swedish manufacturer, Haglöfs, makes a series of hunting packs out of a very soft cotton canvas. This is perfect for traveling through heavy brush in search of game - the pack material it is absolutely silent. The canvas is urethane coated, making it water resistant, but not waterproof. The pack comes with an integral rain cover.
Due to it's lack of waterproofing and ready ability to absorb water or moisture whenever the pack is put on the ground, cotton canvas is not a preferred pack material. Its main advantage is that it is inexpensive material, and packs made of this material will also be inexpensive as a result.


"Pack Cloth" Nylon
"Pack Cloth" Nylon (as it is commonly known in the industry) is a smooth nylon material with threads of equal diameter running in the warp (vertical) and fill (horizontal) directions. It's somewhat shiny and slick to the touch. Many packs utilize this material, and a great number of daypack styles as well. It's not as popular as it used to be, but it's a perfectly good pack fabric. The chief advantages of pack cloth nylon are it's ready acceptance of urethane waterproofing, abrasion resistance, and excellent puncture resistance. The downside is that it can be a somewhat heavy fabric when compared to the other nylons.

Cordura Nylon
Cordura is an "air treated" nylon fabric originally designed by DuPont. It's characterized by a rough, fuzzy texture on the outside, and a somewhat rough texture on the inside. By far, it's the most popular fabric in use today for both backpacks and daypacks. The chief advantages of Cordura fabric are its abrasion resistance and light weight. It's not as puncture resistant as Oxford-weave nylon and doesn't accept waterproofing as well, but it's also a perfectly good pack fabric. (Keep in mind, we're talking about degrees of difference here. Coated Cordura fabric is considered "waterproof" just as Oxford-weave fabric is considered waterproof.)

Rip-stop Nylon
Rip-stop Nylon is starting to emerge in the marketplace, and more companies will probably be using it before long. Rip-stop nylon is easily distinguished by its regular "grid" pattern of heavy threads sewn in the warp and fill directions every quarter-inch. The heavy threads "stop rips" if the pack is punctured or torn. Nylon, when torn, has a tendency to start unraveling. If you're deep in the backcountry with no thread or duct tape, a torn pack can present an interesting challenge in carrying your gear. A torn nylon pack put under stress by 50 pounds of equipment, can quickly start to come apart. The "rip-stopping" design of rip-stop nylon can provide extra protection to help ensure that your pack won't disintegrate in front of your eyes the first five miles down the trail. The main advantages of rip-stop nylon are it's ready acceptance of waterproofing and it's lightness in weight. It's not as puncture resistant as pack cloth nylon, but just about as abrasion resistant. However, since rip-stop is typically used in lighter fabric weights, holes from prolonged abrasion in one spot may appear sooner than pack cloth nylon.

Exotics
Of course, not all packs are made of the four materials outlined above. Manufacturers are always experimenting with new materials. While rip-stop nylon seems to be appearing in more and more upscale packs in the U.S., polyester is starting to make an appearance in some designs. The Lowe "Trekking" series of packs use polyester in the body of the pack, while their "Mountaineering" packs use nylon. Polyester has higher resistance to ultra-violet (UV) degradation. But pound for pound, nylon is still a stronger material, so it's a trade-off. Some companies have been experimenting with Kevlar as a pack material element. Kevlar is typically woven into the fabric to provide tear resistance if a hole should appear in your pack.

Some thoughts on Nylon Material
A second consideration once you've looked over the materials used in packs is the "heftiness" of the material. "Denier" is a measure of the yarn size, or thickness used in nylon material. Daypacks typically use 210 denier cloth, while backpacks may use 420, 500, or even 630 denier cloth. Obviously, the higher the denier, the higher the strength of the fabric, and the higher the weight as well (which can be a consideration....). Most of the higher quality backpacks identify the denier of the material on a cardboard promotional tag hanging off the pack. Does that mean a higher number is better? Not necessarily.

There's an additional factor you should be aware of - tenacity. Basically, tenacity is a measure of the fabric's ability to resist additional tearing once a tear has started. Nylon is available in two types of tenacity. Type 6 is commonly used in packs constructed in the orient and is classified as "low tenacity", or lower strength nylon (tenacity of 3.0 to 6.0 grams per denier). Type 66 is considered "high tenacity" nylon, and is much stronger (tenacity of 6.0 to 9.5 grams per denier). How can you tell the difference? Sadly, there's no way without lab testing

Stitching

You'll find most packs to be sewn with 8 to 10 stitches per inch. The general feeling here is that if the stitching is increased much beyond 10-per-inch, the strength of the fabric starts to degrade. Anything below 6 stitches per inch starts to become suspect in terms of strength. Remember, packs are made of deniers much higher than you'll find in a tent or sleeping bag, and close stitching can actually damage the threads used in the pack material itself.

Zippers
The majority of pack manufacturers are using plastic "coil" zippers on their packs these days. A coil zipper is basically a continuous piece of plastic that's been fashioned into a coil shape and sewn onto a piece of fabric. The chief advantage of the coil zipper is that if fabric gets caught in the teeth, the fabric can be gently pulled out of the zipper without tearing. This is certainly a consideration since your pack will sometimes be jammed with clothing, or on IF packs, with a sleeping bag.

Some EF packs may use regular plastic tooth zippers, and these are certainly okay as well. They won't let go of fabric that easily, but size for size, they're stronger than coil zippers.

Some really inexpensive packs may use metal tooth zippers. (When Kelty first started manufacturing packs, they used bronze tooth zippers on their pockets before the days of the coil zipper.) It's possible that some of the very inexpensive packs may still use aluminum or steel metal zippers, which can either rust or corrode badly when exposed to rain. I don't recommend them.