now for a real answer. A Regualr Mustang has a V-6 engine (older models, up till 1994, had 4-cyl engines like you'd find in an Escort). A Mustang GT has a V-8 engine and dual exhaust, very beefy and nice all around sports car. A shelby is a Mustang GT that's been improved in the suspension, body, and drivetrain, with things like racing shocks, Supercharger, and a more aerodynamic body kit.