Backpacks in general fall into one of four categories: frameless, external frame, internal frame, and bodypack. A pack frame, when present, serves to support the pack and distribute the weight of its contents across the body more appropriately, by transferring much of the weight to the hips and legs.
Most of the weight is therefore taken off the shoulders, reducing the chance of injury from shoulder strap pressure (many backpacks equipped solely with shoulder straps can affect the posture of a person carrying more than 14 kg (30 lbs)), as well as being less restrictive of the upper body range of motion.
Carrry all your things
Most backpacks are capable of being closed with either a buckle mechanism, a zipper, or a dry-bag type closure though a few models use a drawstring fitted with a cord lock for the main compartment.