24Pack Styles
Depending on your individual needs and purposes, there are a variety of backpacks to choose from, including daypacks, internal frame packs, external frame packs and travel (conversion) packs.
Daypacks are used for day trips or general day-to-day activities. Usually frameless, they are designed to carry a lunch, water bottle, camera, some extra clothes and some climbing gear. Sizes and designs can usually vary and personal needs should dictate your selection.
Recommended Features
1. Easy loading from either the top or front panel.
2. Pockets for storing smaller items.
3. Waist strap for stability.
4. Padded shoulder straps.
5. Lash points to offer space for extra gear. 
Internal frame packs are offered in a range of sizes. These packs are good for heavy loads because they transfer much of the weight from your shoulders to your hips. An excellent choice for mountaineering or cross-country skiing, internal packs are designed with a lower center of gravity.

Although not generally as comfortable for carrying heavy loads on a trail as an external frame pack, this design stabilizes the load on your back, allowing for more natural hip-to-shoulder movement and making you more confident in your balance. 

Special internal frame mountaineering packs allow the flexibility necessary for active climbing and skiing. 

Strengths: Slim profile;good mobility and balance for skiing, climbing, etc.
Weaknesses: Less rigid and ride closer to your back than external frame packs; less breathable for your back in warm weather.

Recommended Features
1. Contoured aluminum stays to aid distribution of weight to hips.
2. Lumbar pad to help support and comfort heavy loads and provide more ventilation.
3. Compression straps to help prevent smaller loads from shifting around the pack.
4. Plenty of lash points and loops for extra gear.
5. Slim profile with outside pockets.
6. Shoulder and waist cinch straps to keep load close to your back.
External frame packs are generally large in volume with a rigid support system on the outside of the pack. Designed to carry heavy loads over long distances, they offer excellent weight distribution and a high center of gravity which transfers most of the weight from the shoulders to the hips. Unfortunately, this results in a compromise of your balance, making external frame packs suitable only for backcountry trips on open trails. 
Most external frames are made of aluminum alloy tubing and have either welded or coupled joints. External frame packs come in four shapes: straight, s-shape, hipwrap, and woman's frame.
The straight frame allows for a lot of room between the back and the pack. This makes for great ventilation in hot weather, but it also tends to cause the pack to pull away from the shoulders during hiking.
The s-shape frame is designed to fit the natural curves of the back. This provides for a high degree of balance and stability, while still offering some ventilation to the back.
The hipwrap frame bends in at the waist and wraps around the hips. This helps to transfer weight from the back to the legs. 
The woman's frame is designed to fit a woman's body. It fits shorter bodies, and rides lower on the back to match a woman's lower center of gravity.
Strengths: Good for backpacking with loads of 25 lbs. or more; rigid frame keeps pack separated from your back and is comfortable in warm weather; best for backpacking on open trails.
Weaknesses: Wider profile than internal frame packs and tougher to maneuver in tight situations or where balance is needed (i.e. skiing and climbing).
Recommended Features
1. Rigid, tubular heliarc-welded aluminum frame to support the load.
2. Waterproof coated, top loading pack with overflap or a front-panel zipper load.
3. Sturdy, padded shoulder straps of dense, thick foam with nylon covering.
4. Mesh backband for ventilation in warm weather.
5. Wraparound hip belt made of thick foam with a quick release buckle system.
6. Outside pockets to help organize your materials. 

Travel (conversion) packs are hybrid travel pack/backpacks. Well-suited for traveling convenience or when your plans include both public transportation and the trail, these packs blend a functional piece of luggage with a pack suspension system that can be tucked out of the way for traveling. 
The suspension harness can be a basic pair of padded shoulder straps or a highly technical and fully adjustable design that is suitable for climbing or extended trekking with heavy loads. 
Available in many sizes from smaller carry-on models that fit under an airline seat to models capable of carrying large loads for extended trips. Look for features similar to internal frame packs and daypacks. 
Fanny/lumbar packs are ideal for when you need to carry a few things on a short walk. Worn strapped around the waist, fanny packs are used by some as an alternative to a purse.
High tech versions of fanny packs are also available, offering such amenities as hydration systems. Certain manufacturers offer detachable fanny packs as a part of their pack systems. 
Baby carriers are frame packs designed to carry infants and children. Available in various sizes, some baby carriers feature a simple design for short walks, while others have more complex harness systems for long hikes. Baby carriers are limited to a maximum of 40 pounds.

To choose the right bag for you, you must know how long you are planning to be gone. Use the following guidelines to assist in your selection:

4,000 cubic inches = 2-3 days

5,500 cubic inches = 4-7 days

Cold weather activities may require more space for heavier sleeping bags and extra clothing. However, if you are sharing communal gear like kitchen supplies, you may be able to get away with less.

Beware buying an oversized pack for your needs. If you have the room, undoubtedly you will find gear to fill it, which can make your journey more fatigued and less enjoyable.

Backpack Materials

Most packs are comprised of a variety of  fabrics that are fully waterproofed and designed to combine toughness and lightness. Two of the most popular primary fabrics are Cordura® and Rip-stop,with Rips-top consideredby many to be tougher againsttears. 

High-abrasion areas of a pack are usually reinforced with an even tougher nylon fabric such as Ironcloth®, Spectra® and Pack Cloth®. Most backpacks also have various mesh or ventilating foam cells in the areas that fit the back to increase breathability. 
Fitting Tips

It is important to size your pack to your body. The pack you choose has nothing to do with how tall you are, and everything to do with your torso size. If the pack is too long, it'll sag over your butt. Too short and you'll lose crucial lower back support.
To determine your torso size and find the pack right for you, follow these steps to take your torso measurement.

Locate and Measure Your Torso: The two points that will determine the length of your torso are your seventh vertebra (bony protrusion at the base of your neck) and the top of your hip bones. 

Using a tape measure, measure in a straight line from the seventh vertebra to the point on the small of you back approximately level with the top of your hipbones. Check your measurement against the chart below. 

Torso Length Stay Length Pack Suspension Size
13" -15" (45 cm.)
22 " Stay
16"-18" (45-50 cm.) 
24" Stay
19"-20" (52.5 cm.) 
26" Stay
21" and UP
27" Stay
Extra Large
Please remember, however, that these are general recommendations for pack suspension size. Individual needs may vary.
Test-Fitting a Pack: Knowing your torso length should help narrow your choices. Once you receive your pack, give it a test fitting and adjust as required. Follow these helpful guidelines.
1. Loosen the pack's shoulder straps and hip belt.
2. Slip your arms through the shoulder pads, close and tighten the hip belt comfortably near the top of your hipbones.
3. Make sure the padded areas of your hip belt wrap around comfortably without touching in front.
4. Reposition the buckle pieces if you find the belt too loose or tight. If this does not help, you may need a different pack.
5. Once the hip belt is positioned correctly, tightly cinch down the shoulder straps.
6. Make sure your shoulder straps are spaced enough so that it does not squeeze your neck, but also close enough together so that it will not slip off your shoulders while active.
7. Look for a good torso fit. Simply by loosening and tightening your shoulder straps, you should be able to easily distribute the weight of your pack between your shoulders and hips.
8. Make sure the pack is comfortable. It should feel good on your back and not restrict your movement. You should be able to look up and squat down comfortably.