Smog is a form of severe air pollution. This is made up of smoke and fog and hence called as Smog.
It was coined in the 20th century as blending of the word smoke and fog. This is a condition in which the air pollution is visible and is composed of nitrogen and sulfur oxides, smoke, ozone and some other pollutants.
Man made smog is caused due to pollution caused by vehicles and industrial emission and photo chemical reactions that is caused due to these emissions.