Rucksacks, the size of these varies from day sacks of about 20 liters to expedition sacks of 70+ litres. Generally the larger they are the more they cost. The size you choose depends on what you intend to use it for, if you want to do the Pennine way and carry all your camping equipment then you will need a expedition rucksack. But if your only ever going to do day walks then a large day sack is OK. Remember that it has to be large enough to hold your waterproofs, food / drink, spare clothes, torch, emergency rations and any clothes you take off during the walk. So it is best to get one that is more than large enough. Since its easier to carry a rucksack that is 3/4 full than a full one with a jacket hanging from it which you may lose.
Rucksacks are not totally waterproof so you will need a rucksack liner of some sort this can be a couple of bin liners through to a sealable and buoyant one for canoeists and cavers.
Rucksacks can have compartments in them which generally mean that the bottom third is partitioned off with a zip opening. The idea being that you store water proofs or camping gear in there and keep it separate from the rest of your kit. The main problem is that the zip may break. The compartments aren't really necessary, because with careful thought about what kit you'll need during the day you can pack your rucksack accordingly.