Rucksack liners keep the contents 100% dry, Pack covers don't completely cover the pack so water can still get inside, but they stop your rucksack soaking up rain water and becoming much heavier.
Unless otherwise stated rucksacks are not generally waterproof - most often the fabric is waterproof but the seams will leak in a heavy downpour. Rucksack liners keep the contents 100% dry, buy on half or two thirds the size of your pack because items like cooking gear don't need to be kept dry. Pack covers don't completely cover the pack so water can still get inside, their main function is to stop your rucksack soaking up rain water and becoming much heavier.
