A backpack is the fabric or similar material container that designed to be carried comfortably for extended periods while hiking or backpacking on multi-day trips. It can be a lightweight fabric, frameless pack that is little more than a stuff sack or a more substantial external or internal frame backpack that is designed to comfortably carry heavier loads.
Backpack frames help transfer the weight of the pack to the backpacker’s hips and hip belt. Frameless pack users will typically create a “frame” in the pack using their sleeping pad or other items to create a rigid structure to the pack. Frameless pack users will typically have lower base weights since they are able to carry less weight comfortably than they would be able to with an internal or external frame backpack. Day packs used for day hikes, may or may not have a frame, depending on the style of pack or manufacturer.