The faucet type filters use carbon to remove chlorine, sediment, and things that make your water taste funky, however many impurities don't get removed from charcoal alone. RO
units on the other hand are much more advanced and filter out all the impurities in the water giving you essentially perfect water. The RO
units uses a membrane that is so fine that it filters at the molecular level and only allows certain molecules to pass through the filter. RO
units are slow and have a certain amount of waste water associated with them. For instance for ever gallon of RO
water produced you get 5 gallons of waste water. Many people use that waste water to water plants or even do laundry....the waste water is still better quality then ordinary tap water or well water.
For a more insightful look into RO
do a search on google or yahoo and you can get very detailed explanations.