Here's a small snip it from (a good FAQ on driving manuals):
http://cssdrivertraining.com/manualprimer.htm
Make sure the clutch is depressed. Very important! Some cars have safety switches which do not allow the engine to start unless the clutch is on the floor. If the car is in gear and the clutch is not depressed, the engine is essentially connected to the wheels. Turning on the starter with the key at this point would MOVE the car! (It would also place an unnecessary load on the starter motor, potentially damaging it.) Always start the engine with the clutch depressed.