There are small concentrations of gas components(mainly from industrial and auto pollution) such as NO₂, SO₂ and others that are in air which is mainly N₂ and O₂ (78%N₂ and 21% O₂). Now when NO₂ and SO₂ combine with tiny amounts of water vapor the reactions give rise to NITRIC acid H N O₃ and sulfuric acid H₂SO₄.

When it rains, the rain water will dissolve these acids and this gives us acid rain.

Acid rain is rain that is, well, acid. It has a pH lower than 'normal rainwater'.

Acid rain is caused by the rain droplets literally containing acid within them. The four common acids in the atmosphere are:
Carbonic acid, H₂CO₃, which is formed by the combining of carbon dioxide and water (all rain is consequently somewhat acid),Hydrochloric acid, HCl, which is pumped out by volcanoes, Sulfuric acid, H₂SO₄, which is formed by sulfur trioxide and water, of which sulfur trioxide is belched out of volcanoes and certain industrial uses, Nitric acid, H N O₃, which is produced by nitrous oxides and water, which are themselves produced by unclean burning of compounds containing nitrogen (e.g. coal).