Example of bipolar (BJT) N-channel (NPN) transistor used as a current-driven switching device to turn on an LED. It drives possibly much higher current with much smaller current induced by logic-level circuit (voltage <= 5V), possibly coming from Arduino PWM control pin. Base resistor needs to be calculated carefully to provide desired driving current.
see also my example of MOSFET: http://everycircuit.com/circuit/5224206555414528
