A typical modern day backpack, in its simplest form, consists of a cloth (or other material) that is supported and structured in a way to contain items and secured by a shoulder strap or two.
Today, we have pretty innovative backpack designs, which result in a categorization of backpacks. For outdoor uses, backpacks are generally categorized in one of three types. These different styles include frameless, external frames, and internal frames.
Each one of these styles has pros and cons different than eachother. Below is a short description and some attributes of each of these backpack styles. You can use this information to help guide your decision in purchasing and using your outdoor backpack.
Generally the cheapest, due to less complex design and less material used.
Lightest in weight and smallest in size, making it easy to carry and transport if only needed for small amount of gear.
Unable to store a lot of gear.
Less compatible for extra features (ex: pouches) due to smaller size.
No rigidity and support, pressure can be unevenly distributed on back and shoulders.
External Frame Backpacks
Largest carrying capacity.
Effectively distributes weight and pressure for 'easier' carrying.
Provides spacing bewteen the pack and carriers' back for cooling and ventilation.
Usually the pack can be detached from frame to allow more customized configuration.
Heaviest of all backpack types.
Usually a smaller 'sack' portion than other backpack types.
Internal Frame Backpacks
Semi-rigid, making it flexible yet stable.
Large carrying capacity.
Tight fit, little bouncing and conformity to the body.
Snug to body makes for poor ventilation and 'sweaty' feel on hotter days.
Harder to lash and secure larger and heavier items to outside of pack.