Why is rain normally acidic, what creates acid rain?

Also What is SAV?
THANKS!!

"Acid rain is measured using a scale called "pH." The lower a substance's pH, the more acidic it is. Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3."

http://www.policyalmanac.org/environment/archive/acid_rain.shtml

http://ga.water.usgs.gov/edu/acidrain.html