Substances such as sulphur dioxide and nitrogen oxide are released into the air. As they rise they mix with water and oxygen (and other chemicals that have been released) and form acid rain. These substances can travel via the wind and fall somewhere else in the form of rain, snow, or sleet.

Humans are the cause of acid rain. It is mostly due to the burning of fossil fuels to produce energy. Exhaust is another example of where these harmful chemicals can come from. 

Acid rain is not always a wet deposition. Acid rain can also fall in the form of adry deposition as well. It is estimated that half of the harmful chemicals that are released into the atmosphere come back to the ground in this form. It consists of acidic particles and gases. They are just as harmful to the environment.

The main causes are the presence of sulfuric oxides in the atmosphere which would bond with water molecules in precipitation.  This was mainly due to the heavy emission of these compounds from coal burning powerplants and was partially alleviated by the use of new scrubbing technology in smokestacks leading to a rather precipitous drop in the occurence of acid rain in many places in the world.

The main effects are problems with ph in soils and in water leading to the death of many helpful and important organisms.  There were lakes and some streams that ended up basically becoming completely dead as all the organisms in them expired due to the loss of both important food staples and the death of various organisms.