The simplest form of backpack (also rucksack or knapsack) is a cloth sack carried on one's back and secured with two straps that go over the shoulders and below the armpits. The shoulder is better suited for bearing heavy weights for long periods of time than the hand, so backpacks are often used for that purpose. Backpacks designed for backpacking are considerably more complex.

Backpacking packs, often just called packs, come in two main models: internal-frame and external-frame. In addition to the two shoulder straps, modern packs always have a padded hip belt. The hip belt carries most of the pack's weight, because the pelvis is sturder than the structure of the shoulders; this also lowers the hiker's center of gravity. Packs generally have many pockets, and the main compartment may even have vertical sub-compartments. Almost all packs, especially external-frame models, allow bulky items to be strapped to the outside. Packs are typically about 3 feet (1 m) tall.

Internal- vs. external-frame backpacks

External-frame packs are the older of the two designs and have been in use for at least the past 50 years. An external-frame pack is constructed around a metal (usually aluminium) frame. The frame has a system of straps and pads to keep its metal parts from contacting the body, with the added benefit of improved ventilation and decreased sweatiness. The fabric part of the pack is stretched along part of the frame's length. The main compartment is small compared to that of internal-frame packs, because bulky items (tents, sleeping bags, thermal pads, teddy bears) are strapped to the outside of the pack.

The frame of internal-frame packs is contained entirely inside the pack and consists of strips of either a specially designed polymer or metal that molds to one's back to provide a good fit. Usually a fairly complex series of straps work with the frame to distribute the weight and hold it in place. Internal-frame packs are best suited for skiing and other forms of locomotion involving upper-body movement, but have become increasingly popular for ordinary backpacking as well. In the British Isles, internal-frame packs have completely supplanted external-frame models.

Some sleeping bags come with compression sacks. These are stuff sacks with straps on the outside, enabling the bag to be squeezed enough to fit into an internal-frame pack.