Reverse osmosis (RO) is a purification process that uses a very fine membrane and moves water through it under pressure to eliminate almost all impurities. Reverse osmosis leaves water almost as pure as distillation, but requires no heat or external power source. I have a reverse osmosis filter for drinking water installed under my kitchen sink.
Reverse osmosis is as good as boiling when it comes to bacteria. So, if a warning comes out that local drinking water needs to be boiled, reverse osmosis is an alternative to boiling.
People using water purified by reverse osmosis should be sure to re-mineralize it in some fashion, as the RO process removes healthy minerals as well as unwanted chemicals. Some people feel that it is also important to reactivate a quality of electrical conductivity in the purified water.
You can learn more at http://en.wikipedia.org/wiki/Reverse_osmosis