Backpacks offer an array of features, such as adjustable torso loop, a removable daypack, hipbelt pockets or sleeping bag compartments. A few things to consider:
Frame: Smaller capacity packs like daypacks typically have no frame, while most larger packs have an internal frame that helps support the load and transfer the weight onto your hips so you can carry that weight more comfortably. A few packs have an external frame that is visible outside the pack.
Suspension system: This consists of the frame, hipbelt and shoulder straps-everything engineered to make the pack fit comfortably and carry the load in balanced manner.
Ultralight design: Ultralight backpacks cut weight by using light-yet-strong materials. The lightest packs inevitably trade some durability and structure to save those ounces.
Storage: Most packs load from the top and have a lid with storage pockets. Others load from the side or have bottom sleeping bag compartments. More pockets help you stay organized.
Tool and lash-on loops: Tool loops simplify stowing unwieldy equipment, like an ice axe or trekking poles, on the outside of your pack. Lash-on loops let you strap on other items you don't want to stow inside.
Reservoir compatibility: If you like to hydrate on the go, look for packs with this feature-a sleeve inside to hold a reservoir (sold separately). Hydration compatible packs also have a port for the sip tube.