Acid rain is the name given to any kind of precipitation (rain, snow, hail, etc.) whose pH is less than 5.6, that is, acid.
It turns acid because of the oxides of sulfur and nitrogen it contains, and it is caused mostly by air pollution (smoke from burning fossil fuels, cars and factories).
The effects of acid rain are terrible. It damages the soil, so trees cannot take the right nutrients and they become ill.
It falls in the water of rivers and lakes, with the subsequent pollution of these waters and the fish and other creatures living or making benefit from them. It damages monuments, buildings and statues. In short, it destroys a whole ecosystem.