Many cartridge-type filters use just a paper or fiber media to trap particles. This is the least-effective form of filtration. Depending on the porosity of the media, very small particles can be trapped. But chemicals like chlorine, and other undesirable taste and odor impurities may not be removed.
Reverse-osmosis water filters produce pure water by forcing untreated water or tap water through a semi-permeable membrane. The membrane lets the water molecules pass through, but traps the impurities--and often some desirable minerals.
Reverse osmosis is quite a slow process for a residential-grade filter. It may produce only a few quarts of pure water per day.
Note… Don't use a reverse-osmosis water filter on an ice maker line. They don't generate enough water pressure to operate the water-inlet valve properly.
Most residential water filters use activated-charcoal filters. These are used in icemaker filters, under-counter filters, countertop filters, whole-house filters, and so on.
A charcoal filter works on the principle of adsorption. Large volumes of gases, including most poisonous ones, adhere to the charcoal. Charcoal--which is so effective that it's also used in gas masks--is quite porous. Its porosity gives it a very large "surface area" with which to adsorb a lot of impurities.
Charcoal, which is mostly pure carbon, results from the partial burning or destructive distillation of organic material. With additional special heating or chemical processes, charcoal's adsorptiveness can be greatly increased--making what we call "activated charcoal."
