Air pollution is caused by the fumes that are given off from factories. Factory fumes contain different chemicals that were results from chemical reactions occurring in the factory.
Another factor is the fumes from cars. Cars give off carbon monoxide (CO) which is another chemical that causes air pollution. Carbon monoxide also can not be used by plants because of it's missing elemental factor: another Oxygen atom (making it CO2).
Essentially anything that gets into the air, that isn't air, can be considered air pollution. That can range from volcanic emissions, to gas emissions from cattle, to smoke from power plants, to dust kicked up by plowing fields.
Because the causes are so varied, so are the effects. They can range from acid rain killing fish, to children getting asthma, to ozone depletion.