The carbon filters use a carbon cartridge that allows water to get though holes in the surface (a porous surface). As the water runs through it, the carbon filter absorbs many substances that are not supposed to be in water. This catches the Ca and Mg that make water hard, so one the water has passed though the filter, it becomes soft (opposite of hard water). The water becomes more soft the longer it has been in contact with the filter and on how much carbon there is in the filter. These filters have charcoal in them, and the more charcoal there is, the more carbon there is.

There are even different types of carbon filters, specifically made for certain purposes and places. The faucet-mounted filters are attached to a faucet so that the water flows though it as it comes out of the faucet. The portable filter are put on containers so that as you pour water out of the container, it becomes filtered. There are also specialty-type filters that you can attach to water lines that go to your refrigerator or an ice maker. Another type is a line by-pass filter that can be installed beneath the sink or on a water line that enters the house. The fifth type is a stationary water filter that goes in a cold water line underneath the sink. There, all cold water that goes though the pipe into the kitchen becomes filtered.

This is an example of a faucet-mounted filter.
http://www.cqwater.com/show.cgi/JSHENTERPRISES/howitworks.htm

The second main type of filters are make of fiber. The fiber contains spun cellulose or rayon that are wrapped around a cylinder near the opening. Pressure forces the water though the fiber before letting it reach the faucet. the fibers trap silt and other unwanted minerals in the water.

The third type is a reverse osmosis unit. These work by forcing water though a synthetic membrane that is semi-permeable (partly able to let things though). This separates the minerals that have been dissolved in the water from the water. The name comes from a process called osmosis where a membrane separates two solutions, one which has more substances in it than the other solution. Pressure is put on the membrane to let the substances flow though both of the solutions, creating an equality in the amount of the substance in the solutions. This type of filter does the opposite of this because it is taking the substance from a solution.
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