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.