Everyone knows thunder as the sound that comes after lightning, but there's many things about thunder that people don't know. Something that many people don't know is that thunder and lightning occur at the same time, it seems like lightning comes first because light travels faster than sound. Thunder is caused by the heating of air by lightning. The lightning heats the air up to as much as 60,000 degrees Fahrenheit when air is heated quickly it expands and contracts violently causing an explosion which causes sound know as thunder.
http://weathereye.kgan.com/cadet/lightning/thunder.html
http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tstorm/tstorm_lightning.html
http://www.loc.gov/rr/scitech/mysteries/thunder.html
