There are two main types of filter media in a Brita filters: Carbon granules, and De-ionization resin beads (DI). The DI resin beads remove charged ions and radicals from the water. These include calcium, magnesium, sodium, carbonate, and any other dissolved solids with a charge. The carbon granules reduce chlorine compounds and a number of other organics and inorganics. This is probably the most useful part of the Brita filter.
Brita filters also eliminate 98% of lead and reduce copper, chlorine (taste and odor) and mercury, all of which may be found in tap water. This gives ordinary tap water a much better/cleaner taste.
Another very useful method for a brita filter is using it to purify cheap vodka. Using a Brita filter, one can turn cheap, off-brand vodka into smooth, tasteful, inoffensive spirits that regularly win taste tests over name brand vodkas. Simply pour cheap vodka through the filter four times to make it smooth and delicious.
Dude, your vodka tastes like ass! Lets run to the store and pickup a Brita.