Conventional Regulators
Older versions of conventional regulators were quite bulky to look at. A square box jutting out of a board with a circular knob or toggle switch did not make for very sleek appearance. The box contained the circuit elements of the regulator system. You had to adjust the knob to set the fan at the desired speed. Modern day conventional regulators present just a toggle on a board that incorporates the switches for the other electrical devices for the room. You do not get to see the regulating unit concealed in the wall behind the board.
To understand how a regulator works, you must know something about resistances. Any electrical conductor allows current to pass through it. The conductor however, offers a certain amount of resistance to the passage of current. The resistance depends upon the material of the conductor.
The regulator has spools of wire with different amounts of resistances. When you set the knob at a particular position, you include a certain resistance in series with the fan. A series connection implies the resistance is in line with the fan. This reduces the voltage drop across the fan and its speed to your desired level. The greater the resistance, higher is the voltage drop across it and that lowers the speed of the fan.