I want to simply understand how does the DC drive controls the speed of the DC motor and it would be great if someone would suggest a good website that i can read more about stuff like PID/IGBT/Thyristors, etc.
As most modern DC motors use permanent magnet fields, the speed at which the motor runs is determined by the armature current. A common method of achieving this is to use a variable voltage supply to the armature but these variable supplies are wasteful of power as much is lost as heat from the series regulator. The more acceptable method of control is to use PWM (Pulse Width Modulation), this is achieved by switching the supply on and off many times a second. The power wasted as heat is minimised as is the size of the series control semiconductor. These PWM controllers find use in any DC drive control especially when batteries are used. Your battery operated drill has a PWM controller.