NPN Circuit

When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off

PNP Circuit

Known as a Bipolar Transistors Switch
The secret to making a transistor switch work properly is to get the transistor in a saturation state. For this to happen we need to know the maximum load current for the device to be turned on and the minimum HFE of the transistor. For example, if we have a load that requires 100MA of current and a transistor with a minimum HFE of 100, we can then calculate the minimum base current required to saturate the transistor as follows:

Minimum base current = 100 MA / 100
Minimum base current = 1 MA

In actual practice, it is best to calculate about 30% more current than we will need to guarantee our transistor switch is always saturated.  In this case, we will use 1.3 MA. We must also select our supply voltage, so for this example we will use 12 volts. We can now calculate resistor R1 in the circuit as follows:

Maximum Current Required = 100MA
Supply Voltage = 12 Volts

R1 = Supply Voltage / ( Maximum Current Required / Minimum HFE * 1.3 )
R1 = 12 / (.1 / 100 * 1.3)
R1 = 9230.7 or 10K for nearest standard value.

R2insures that the base of the transistor does not go slightly negative which would cause a very small amount of collector current to flow. The value of this resistor is not critical but a value about 10 times R1 is normally chosen.

To turn on our transistor switch all that is needed is to short resistor R1 to the negative ground.