Bring a minimum of 1 gallon per person per day. If your campsite has potable water, plan to bring enough for your hike in. Always carry at least one backup purification method.
Purification by Boiling
Bringing water to a full boil--for at least one minute, preferably a little more just to make sure---kills pretty much everything in the water and is the best water purification method.
Chemicals such as iodine or chlorine tablets work but might not be as effective as boiling and can adversely affect water taste.
Filters create potable water by forcing the water through a filter medium, usually charcoal. Filters often need to be augmented with a chemical system because while the filters will trap lurking micro-organisms, they generally can't catch something as small as a virus.
Pre-positioning potable water at strategic points along your route works if your travels take you through areas such as the desert where water is absent.