A fan controller is simply a tool that controls the voltage of a device in your computer, usually a computer case fan or a CPU cooling fan. By turning down the voltage on the fan, the fan spins slower and makes less noise. If you have a noisy case fan, you may be able to silence the fan by adding a fan controller to the fan.
Fan Controllers = Voltage Regulators = Fan Resistors
Fan controllers, whether controlling single or multiple fans, can simply control the fan speed. Voltage regulators and fan resistors are simply other names for a fan controller. In some cases fan controllers can also monitor the temperature of the fan and various locations inside your PC. Using a front-mounted fan controller with temperature monitoring allows the quiet computer user to produce the quietest computer possible, while still maintaining a sufficiently cool environment for their PC components.