In this tutorial,
The author makes this weird(to me apparently) connection of 5V pin of arduino to 12 V power supply, and explains itself by:
You need to have two power sources - one for the Arduino, and a separate 12V power source for the motor. You cannot connect the Arduino's barrel jack to the 12V! This will create a common ground between your Arduino and the 12V power supply. And it would fry the Arduino when you are creating the common VCC needed for this circuit. (With an N-Channel MOSFET you don't have this problem since you want to have a common ground between the power source and the Arduino)
However, I really did not understand why connecting grounds of two power supplies together and then applying HIGH or LOW to the Gate of the p-channel MOSFET does not do the trick.
So, a little bit help on understanding what is going on would be appreciated.