Chlorine dissipates or out gases from water rather quickly on its own. The common practice for aquarium owners to set some tap water aside for few days before using it works fine when chlorine alone is present, but not so with chloramines.
Chloramines are a combination of chlorine bonded with small amounts of ammonia. It is not unusual for water supply companies to convert to the use of chloramines to allow them to meet stricter state and federal water quality regulations, but also because chloramines have the ability to last in the distribution system, often taking weeks to disappear from the water. As an added benefit, chloramines have virtually no odor or taste.
When dealing with chloramines in tap water it's of the utmost importance to the safety of aquarium fish and all other aquatic animals that the water be treated properly, not only to remove the chlorine, but the associated toxic ammonia as well.