Waist packs are worn around the waist instead of over the back, usually consisting of a small pouch attached to a belt. The weight is distributed around the waist to keep from straining the body while carrying. Most wait packs have side pockets for holding drinking bottles. They can carry loads of up to ten liters.]
Hydration packs are designed to provide easy access to drinking facilities. The backpack is usually built around a water bladder with a drinking tube extending from it. Some models consist purely of the water bladder, while others have side pockets and other compartments.
Day packs are relatively small and are designed for daytime hikes. They are supported entirely by shoulder straps with no hip belts, although some models also have chest straps to keep the shoulders straight. They have capacities of 15 to 35 liters.
Midsize packs have much higher capacities and less volume and weight. They are ideal for multi-day hikes without much cargo volume. They are also great for recreational hiking, with facilities for carrying additional items like books and cameras. They can carry loads from 35 to 70 liters.
Expedition backpacks are full-sized packs designed for long hikes spanning several weeks. They have a capacity of 60 liters or higher. They usually come with broad hip belts to distribute the weight more evenly around the body, and a large lumbar pad to keep the back comfortable.