A backpack is a container that is strapped onto a hiker's back using straps. There are a multitude of types, sizes, materials, and functions of backpacks depending on the intended style of hiking.
Carrying Capacity
The carrying capacity of a backpack is measured both in size, which is typically measured in Liters (L), as well as maximum weight. Both need to be considered when purchasing a backpack, and as such many experienced hikers recommend that all other hiking gear should be purchased before the backpack. This way the hiker will know exactly how much weight and volume they need to carry with them. There is an old adage that says that a hiker will fill their backpack if there is any extra space, causing to unnecessary weight, so ultralight hikers prefer to leave as little extra space as possible to prevent them from the temptation of filling their pack with extra luxury items or conveniences.
Frames
A frame in a backpack is a mechanism that helps distribute the weight of the load being carried in the pack across the hiker's body in a more comfortable and efficient way. Nowadays, the majority of larger packs are Internal Frame, meaning that they have a frame inside the pack. In the past, the most common types of packs were External Frame, meaning that the frame of the pack could be seen on the outside of the pack itself. Frameless packs are the third type of framing for a pack, meaning there is no frame at all, inside or out. Frameless packs are the most common style of small daypack, but are also found as larger capacity Backpacking packs, used by Ultralighters. Frameless packs are generally less comfortable for carrying heavy weights, but are a lesser weight overall than a pack with a frame. As such, Ultralight backpackers will often opt for this style of pack because their gear is generally not very heavy, and they are looking to save on weight wherever they can.
Materials
There are a multitude of materials commonly used in hiking backpacks, including nylon, polyester, DCF (Dyneema Cuban Fiber), Robin, polypropylene and canvas. The type of material used for the backpack will depend largely on the purpose of the pack, and the price point. Some materials are optimized for durability, while others may be optimized for weight savings, waterproofing, or cost savings.
For Day Hiking
Backpacks that are designed for day hiking are typically referred to as daypacks, though there is not a firm definition as to what size constitutes a day pack. These packs will typically be small, generally between 8 and 32 liters, though they are sold larger or smaller. Daypacks typically don't have some of the added functionality that comes with backpacking packs, and are streamlined for convenience. Most of them will be frameless, though some of the larger capacity daypacks may have an internal frame.
For Backpacking
Backpacks designed for overnight backpacking trips will generally be much larger than day packs. These can be anywhere from 20 liters to 90 liters, depending on the style of the hiker. Ultralight hikers will opt for smaller capacity packs without a frame, as they bring less gear with them on their trips. They may have additional components such as hip belts, sternum straps, load lifters, a brain / lid, compression straps, trekking pole attachments, ice-axe attachments, external attachment points, and more.
Hip Belts
Hip belts are essential for hikers wishing to carry a heavy load on their backs, such as overnight backpackers, long distance hikers, or thru-hikers. The hip belt is a belt that clips together and tightens around the hiker's hips, transferring the weight of the pack onto the legs instead of the back. Hip belts are very effective at increasing the weight a hiker can carry and the distance they can go because it is believed that the human body has evolved and optimized to carry weight on the hips, and not the upper body. Hip belts can also have pockets for additional storage and convenient access that is sought after by long distance hikers.
Sternum Strap
A sternum strap is a strap that comes across the chest of the hiker, and clips together to help keep the pack close to the body. Contrary to what many beginner hikers believe, the sternum strap is not intended to be tight, but relatively loose, and only there to prevent sway and movement of the pack. This strap is usually attached at both of the shoulder straps.
Shoulder Straps
The Shoulder Straps are the main large pieces of fabric that run overtop of the hiker's shoulders, and down across their upper chest. These straps can vary greatly in the thickness and padding, trading weight for comfort. Often times these straps have sternum straps and load lifters attached to the front and top respectively, and can be adjusted depending on the needs of the hiker. There are customizable attachments that many hikers like to use on their shoulder straps, such as water bottle holders, camera attachments, carabiners, and more.
Load Lifters
Load lifters are straps that come overtop of the shoulder straps of the pack. The hiker would pull these straps to tighten the load lifters, which would bring the weight of the pack closer to the spine of the hiker, making the weight closer to their center of gravity and providing a phenomenon of a lighter pack. Not all packs have load lifters and they are seem by some hikers as unnecessary.
Backpack Brain / Lid
A backpack brain or lid is a pouch that covers the main opening of the backpack, and usually has a separate compartment where items can be stored for easy and convenient access. The brain will sometimes be detachable altogether, allowing for more versatility in the backpack.
Compression Straps
Compression straps are any straps on the backpack that can be tightened or adjusted. Generally these are to compress the pack down when the volume is not entirely full, or to compress the gear inside the backpack into a more compact unit for better weight distribution. Backpacks will often have many different compression straps in different locations on the pack. Sometimes these are cut off by ultralight backpackers in an attempt to go to extreme measures to save weight.
Waterproofing
Most backpacks are not waterproof, though some are. Generally DCF (Dyneema Cuban Fiber) packs will excel in waterproofing, with the trade-off being an expensive price tag. Most hikers will opt to forego this material and go with a cheaper fabric, and use a different means of keeping their gear dry. A lot of backpacks will come with a pack rain-cover, which may or may not be integrated into the pack with it's own compartment. Many hikers have varying degrees of success with these types of covers. Because keeping gear dry can be a life or death situation, may will choose to use a pack-liner, or simply a plastic trash back o the inside of their pack in order to ensure that the essential gear, such as their sleeping bag, will not become wet and create a miserable or even dangerous situation for them on their trip.
Zipper vs. Rolltop
The two main ways that hiking backpacks are closed are by zippers and by rolltops. Zippers are the same as a zipper you would find on clothing, however sometimes they are done with a special water-proof sealing to prevent water from seeping into the pack. Many prefer zippers because they are a quick and simple way to access the contents of the pack. Another common type of enclosure is the rolltop, which is a long type of fabric towards the top of the pack which can be rolled up and then clipped closed. The benefits to this style is that the pack can store a higher volume when it is needed, or rolled down to make the pack smaller when it is not needed. These are also the style in which most water-proof packs are made, because they roll in on themselves and do not have a zipper as a weak-point to the waterproofing. Another, less common type of enclosure for a hiking backpack is a draw-string, which is most common in very small and lightweight daypacks.
