Acid rain is where chemicals in the air combine with rain and make the water more acidic.
Normally, the pHvalue of rain is about 5.6, which means it is a weak acid. Other chemicals can make this a stronger acid if they react with it. When this rain falls to the ground, it can kill fish in the rivers, harm plants, and dissolve building materials. This is why many gravestones are unreadable.
The most common chemicals which do this are oxides of sulphur and nitrogen, which are put into the air by power stations and vehicles (such as cars). There are also some natural causes, like volcanoes and biological processes that happen on the land, in wetlands, and in the oceans.
Many countries are trying to reduce the amount of acid rain by agreeing to treaties, and by taking part in emissions trading. In the United States, many power stations use a process called Flue gas desulphurization (FGD) to reduce the sulphur in the gases that are made.
Acid rain can make forests grow slower, or even destroy them. This affects animals and their natural habitats. Acid rain can also pollute a water supply, and this may not be noticed because the water will look the same. It would also hurt or in some cases it has been known to burn people and kill small mammals such as cats and dogs and mice. It could be a contributing factor to other diseases such as Cancer and Alzheimer. It could corrode metals such as TV aerials, cars, but that will have to be very very strong acid.
See also: Acid Rain K-12 Experiments & Background Information
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)