At around 60+ litres, these packs are amongst the largest available and are perfect for heading into the wilderness for a few nights on a self-supported trip.
Main features include a large padded hip belt and chunky shoulder straps, which help to support and efficiently distribute the heavy load. They will have a large internal section for the majority of your items, as well as internal and external pockets for storing gear that you may want to access more easily.
Multi-day packs often have a variety of external straps and buckles. Some of these allow items like a tent or a roll mat to be strapped to the outside, whereas others are to compress and stabilise the load. Take time to familiarise yourself with these straps as although they may look alike, they have very different functions - and you don't want to be caught out in the wild.
With a large, heavy rucksacks it is very important that it fits you correctly to avoid getting injured.