Shop around; you will have choices in price, installation, maintenance, and warranties. Be wary if a salesperson claims a device has governmental or EPA approval. The EPA doesn't test or approve products, it only registers them. For a listing of certified units and the contaminants they remove, call the independent testing organization NSF International at (313) 769-8010. You can locate certified dealers by contacting the Water Quality Association at (312) 369-1600.
If the only problem with your water is grit, dirt, sediment, rust, or other such particles, a physical filter may be sufficient. Made of fiber, fabric, ceramic or other screening media, these simply catch particles--including, in some cases, small organisms like cysts and some bacteria. Don't rely upon them to handle disease-causing organisms, VOCs, metals or the like.
If your water tastes, smells, or looks bad, a filter containing activated carbon (AC) may solve the problem. If you want to remove chlorine, pesticides, herbicides, radon, trihalomethanes (THMs) and some inorganic chemicals, carbon may do the trick, too. Some --but not all-- carbon filters are effective at reducing lead content. Check the certification of the unit you're interested in buying and, if claims are made regarding lead removal, be sure they're substantiated.
You can't rely upon a conventional carbon filter to remove salts, nitrates, nitrites, or some metals. And you shouldn't rely exclusively on one to remove organisms. In fact, be aware that a carbon filter will accumulate the contaminants removed from water and bacteria may even breed in it, so you must replace filter cartridges religiously, according to the manufacturer's recommendations.
A carbon filter that contains pesticide silver may be registered--but is not endorsed--by the EPA. Studies show that these are not fully effective at eliminating bacterial growth or microbiological contaminants. Again, be sure any such claims are substantiated.
Effectiveness of a particular carbon unit is a factor of the amount of activated carbon it contains. Although you can buy beneath-the-counter, countertop, and faucet-mounted styles, the $30 faucet-end models are only marginally effective. Whole-house systems are also available for $3000 or more. Such a system, known as point-of-entry (POE) is recommended for applications where a contaminant such as radon poses a threat to the entire house.
If you must remove inorganic chemicals, such as salts, metals (including lead), minerals, nitrates, asbestos, and some organic chemicals, consider a reverse-osmosis (RO) filter. Actually, most models include carbon pre-filters and post-filters, too, which will catch sediment, pesticides, herbicides, THMs and radon.