What's the Difference Between Filtered Water and Tap Water?

If tap water tastes like chlorine, filtering can fix that. Filtering can clear out many safety concerns like bacteria, heavy metals and pesticides. Some types of filters can do nearly everything. Others are specific to certain contaminants.

The most common types of filters in home treatment systems include:

Activated carbon -- The most common type of filter and a relatively inexpensive option (it's used in most of those countertop pitchers), activated carbon attracts and absorbs particles. Water runs through a filtering screen containing carbon, where contaminants get stuck. It can remove much of the heavy metals, parasites, pesticides, radon and MTBE that may be in the water. This filtering is typically used in point-of-use devices, like under-sink or faucet-mounted units and pitchers.
Aeration -- Typically used at the point of entry, aeration filtering forces water entering the home to pass of high-pressure air jets. If there are fuel byproducts or radon -- contaminants that easily become gases -- in the water, they evaporate. Aeration doesn't remove other contaminants like parasites or mercury.
Cation exchange -- This is a water-softening filtering approach that uses positively charged particles to attract negatively charged particles (ions), such as calcium, magnesium and barium. Water flows through beads of resin, where the positive ions stuck to the beads trade places with the negative ions in the water. Calcium and magnesium aren't really safety concerns, but they can damage a home's pipes. However, barium can be a health concern.
Distillation -- Boiling water is one of the best ways to remove pathogens and heavy metals, and distillation takes this route. Distillers boil water into steam and then condense it back into water, killing bacteria and viruses and pulling contaminants like lead, mercury and arsenic out of solution in the process. It's often used in point-of-entry systems (where the water enters the home) or in countertop devices.
Reverse osmosis -- One of the most effective and costly filtering methods and typically a point-of-use approach, reverse osmosis uses pressure to force water through a semi-permeable membrane, removing practically all contaminants. This type of filtering is recommended for people with compromised immune systems, since it tends to eliminate more pathogens than other methods. The downside is the waste: About 4 gallons (15 liters) get thrown out for every 1 gallon (3.7 liters) of purified water it produces.
UV Disinfection -- UV disinfection destroys parasites, bacteria and viruses with ultraviolet light. It doesn't remove other contaminants like metals or chemicals from water. UV is often found in point-of-use, under-sink systems.

All of these methods can be effective at purifying water, but that's not the end of the story. Water-filtering systems come with their own set of problems.