Chlorine: Municipalities inject chlorine into the water supply in order to keep bacteria from growing. It's a necessary form of treatment to keep the water supply safe from pathogens. Typically, the level of chlorine found in tap water is about one-third of what's found in a swimming pool. Chlorine, of course, smells and tastes unpleasant.
Trihalomethanes: Chlorine interacts with organic material found in water, and these byproducts are known as trihalomethanes, or THMs. THMs are considered a factor in cancer rates. The typical level of THMs found in New York City water is generally close to the level established by the Environmental Protection Agency as its Maximum Contaminant Level (MCL).
Metals: Copper, iron, manganese and zinc are often found in tap water. These are not necessarily health hazards, although they certainly affect taste and discolor water. Lead is less commonly found; it is considered to exceed the EPA's Maximum Contaminant Level at even 15 parts per billion.
Turbidity and Particulates: Technically, the turbidity level of water is an optical measure of unspecified particulate matter. In New York City, most particulates are actually oxidized manganese and iron that come out of solution as the water reaches its destination. (The turbidity level of New York City water is usually close to the EPA's Maximum Contaminant Level.)
Cryptosporidim: This is a parasitic cyst, which, because of its shell, is not killed by chlorination. It is episodic, meaning that at any moment it may or may not be present in tap water. It is considered especially threaening to individuals with weakened immune systems – those who are, for example, HIV positive, undergoing chemotherapy, elderly or infant.
For specific water analysis reports from your area, including comments, context and definitions: What are You Drinking?