“You can live three weeks without food, three days without water, and three minutes without air.” – anonymous
“If you have water you have 1000 problems. If you don't have water you have 1 problem.” – anonymous
“…you really appreciate every drop of water if you have to schlep it.” – gcal1)
“The problem is water…” – Mark Watney, while stranded on Mars
Water is critical for life, especially in hot/dry areas like deserts.
Vandwellers living minimally (no showers or slip-n-slides) will typically require 1 gallon of potable water every day for drinking, and 1 additional gallon for cleaning, hygiene, cooking, etc.
Cleaning water to be suitable for use has two components; filtration and purification. Filtration means removing particulate matter such as mud, sand, large plant cells such as algae and phytoplankton. Purification means removing or killing the germs and bacteria that can make you sick. Many methods will do both, but not all.
Not all water needs to be both filtered and purified. Some natural water from fast-flowing streams or springs may have very little particulate matter in it and be very clear already, and will only need purification if you're going to drink or cook with it.
As a general rule, filtered water is fine for showering or cleaning with. Purified water is needed for drinking and cooking.
Not all water can be made safe for use; it is not practical to purify water that is chemically or mineralogically tainted.
See this guide from the National Park Service about how to treat water. Also see REI's guide that wants to sell you stuff (But is still useful).
It is far easier to use less water than to haul, store, and dispose of gray water. Each subpage will have a conservation section.
Potable water is water that is know-safe for human consumption. Municipal water supplies and water kiosks generally fall into this category.
If the water is non-potable (see below) or not known to be potable, you might purify it or use it for non-consumption purposes.
Water for cooking has lower requirements for biological purity because the water is usually boiled during preparation (see above). See this article about using less water when cooking.
Water used for cleaning or external hygiene may be of even lower quality assuming one is careful not to ingest the water, or get it in open sores or mucous membranes.
Washwater (non-sewage waste water) is usually stored in a gray tank for later handling.