A diesel engine uses the heat of compression to initiate ignition to burn the fuel.
In a petrol engine the fuel/air mixture is ignited by a spark plug.
Spark plugs each have a large black wire running to a plastic cap on top of them - on the top of the engine...
http://media.photobucket.com/image/spark%20plugs%20206/SAB206/Spark%20plugs/IMG_0401.jpg
Peccavi, November 2009