Types of Rucksacks/backpacks

Backpack designs
Backpacks in general fall into one of three categories: frameless, external frame, and internal frame. A pack frame, when present, serves to support the pack and distribute the weight of its contents across the body more easily (generally by transferring much of the weight to the hips and legs), so most of the weight does not rest on the shoulders, restricting range of motion and possibly causing damage from pressure on the straps. Most are capable of being closed with either a buckle mechanism or a zipper, though a few models use a drawstring for the main compartment.

Frameless
The simplest backpack design is a bag attached to a set of shoulder straps. Such packs are used for general transportation of goods, particularly by students, and have variable capacity. The simplest designs have one main pocket combined with webbing or cordage straps; more sophisticated models add extra pockets, waist straps, padded shoulder straps, padded backs, and sometimes even reflective materials for added safety when the wearer is out at night. Such packs can be made inexpensively.
Some outdoors packs, particularly those sold for day hikes, ultralight backpacking and mountaineering are sometimes frameless as well.

External frame packs
The more traditional type of frame pack uses a rigid external frame which is strapped on the back and in turn carries and supports a cloth or leather sack and potential strapped on items. External frames were traditionally used to carry heavy loads (20 kg / 40 lbs and more), giving the wearer more support and protection and better weight distribution than a simple, frameless strapped bag. Wooden pack frames have been used for centuries around the world (Ötzi the Iceman may have used one in Copper Age Alpine Italy though some archaeologists believe the frame found with the body was part of a snowshoe), and such gear was common in military and mountaineering applications right up to the 20th century metal versions first appeared in the mid-20th century, and plastic designs towards the turn of the 21st. Modern pack frames are usually made from lightweight metal tubes, generally aluminium but sometimes also using titanium or scandium alloys. The frame typically has a system of straps and pads to keep the sack and the frame from contacting the body. The open structure has the added benefit of improved ventilation and decreased sweatiness. The fabric part of the pack occupies part of the frame's length, but the frame typically protrudes above and below. These areas of the frame allow bulky items (such tents, sleeping bags, and thermal pads) to be strapped on. Thus the main compartment is smaller than that of an internal-frame pack, because bulky items (tents, sleeping bags, thermal pads) are strapped to the parts of the frame not occupied by the main compartment itself. This may result in a less smooth load (annoying in dense forest) and less control over the movement of the center of gravity of the pack. While less popular than internal-frame gear, some manufacturers (such as Kelty, Jansport, and Coleman) continue to produce external packs, and military packs are often external-frame designs as well.

Internal frame packs
An internal-frame pack has a large cloth section in which a small frame is integrated. This frame generally consists of strips of either metal or plastic that mold to one's back to provide a good fit, sometimes with additional metal stays to reinforce the frame. Usually a complex series of straps works with the frame to distribute the weight and hold it in place. The close fitting of the back section to the wearer's back allows the pack to be closely attached to the body, and gives a predictable movement of the load; on the downside, the tight fit reduces ventilation, so these type of packs tend to be more sweaty compared to external frame packs. The internal construction also allows for a large storage compartment. Internal-frame packs may provide a few lash points (including webbing loops and straps for sleeping bags and other large items), but as the frame is fully integrated and not available on the outside, it is difficult to lash a large, heavy item so that it stays fixed and does not bounce, so most cargo must fit inside. Internal-frame packs originally suffered from smaller load capacity and less comfortable fit during steady walking, but newer models have improved greatly in these respects. In addition, because of their snug fit, they ride better in activities that involve upper-body movement such as scrambling over rocky surfaces and skiing. The improved internal frame models have largely replaced external frame backpacks for many activities.

RuckSacks/Backpacks

Backpacks are sometimes used as luggage, particularly as carry-on bags for airplane travel.
Backpacks form an essential part of the gear of the outdoor trekker and the urban backpacker, allowing more mobility and compactness than would be available to someone carrying most of their gear and clothing in a suitcase.
In addition to their use in outdoors pursuits, backpacks are sometimes used in other sports as well. Hydration packs, sometimes used by runners and bicyclists, carry water (in either a bladder or a rigid bottle) and have a tube connected to them from which the wearer can drink without removing the pack; this feature is also included in some more general-purpose hiking backpacks. Backpacks that carry skateboards have also become more popular in the youth culture.

Backpacks for outdoor activities

Large internal-frame backpack
One common special type of backpack (sometimes referred to as a "technical pack" or "frame pack") is designed for backpacking and other outdoors activities. These type of packs are more complex than most other backpacks. Compared to backpacks used for more day-to-day purposes such as schoolbooks, such packs are designed to carry substantially heavier loads, and as a result most such packs attach not only at the shoulders but at the hips, using a padded hip belt to distribute the majority of the weight of the pack to the legs and not the back. The often heavily padded and sometimes semi-rigid shoulder straps are mainly for balancing the weight. They usually (except for those used in ultralight backpacking) have a metal or plastic frame to support and distribute the weight of the pack. Larger packs of this type tend to have a subdivided main compartment. These trekking packs often have several pockets on the outside; they may also have lash points on the exterior (either directly attached to the frame or webbing loops), so that bulky items may be strapped on, although depending on the pack design and type of trek most backpackers will try to stuff everything into the pack. Multiday packs typically have a content between 60 and 100 liters (and are about 3ft /1 meter tall). Smaller packs with similar features are available for shorter trips.
The most common materials for such packs are canvas and nylon, either ripstop fabric for lightweight packs or heavier fabric such as cordura for more typical usage. Most such packs are purpose-designed for the outdoors market; however, it is not uncommon for military surplus packing gear to be sold to outdoorspeople as well for the same purpose. The cheaper versions of the outdoor packs are often favoured by city trekkers; as they have a large volume and still carry relatively easily.
Outdoors packs, in addition to the distinction between external-frame and internal-frame, can be further subdivided based on the duration of trip a pack might be expected to be used on; daypacks hold supplies for a single day's hiking (size about 20-30 litres), while "weekender" bags can hold two to three day's worth of gear and supplies (sizes about 40-50 litres). Larger packs generally have no specific names but are designed to distribute the weight of increased numbers of gear and supplies for longer-duration trips (60-100 litres); such packs often include complex ergonomic support features to simplify the carrying of large amounts of weight. A third type with little or no frame at all, similar to the bookbags used by students and made of light fabric (often nylon ripstop, as mentioned above), is used in ultralight backpacking to eliminate the weight of the frame and heavy fabric used in more typical outdoors packs. Despite (or perhaps because of) their lesser weight, such packs are seldom less expensive than more typical, regular-weight packs.