A transistor will not conduct so that no current will flow from the collecotr to the emitter unless there is a potential difference between the emitter and the base of 0.6 V or above. This property allows the transistor to be used as a switch, which is on the base emitter voltage is 0.6 V or above and off when the voltage is less.
A transistor being used as a switch is shown above. When the transistor is off so that there is no current flowing through it, the output signall is high. As soon as this the transistor starts to conduct, the output voltage falls to zero. This means that the out signal is low when the input signal is high and vice versa. The transistor above is being used as a NOT gate.