What would cause sparks and flames to come out of the tailpipe of your car?

The engine is getting more fuel than your ingnition system can light causing raw fuel to be dumped into the exhaust.I would suggest you have a mechanic check for fuel injectors staying open,if that is not the problem have an analysis done on your ignition system to check primarily your spark.Make sure you get your vehicle checked as soon as possible,this is extremly dangerous.