Bipolar Junction Transistor (BJT)

A Bipolar Junction transis (a.k.a. a BJT or Bipolar Transistor) is an active semiconductor device formed by two P-N junctions whose function is amplification of an electric current
Bipolar transistors are made from 3 sections of semiconductor material (alternating p-type and n type), with 2 resulting P-N junctions. Schematically, a bipolar transis can be thought of in this fashion
There are plenty of texts around on basic electronics, so this is a very brief look at the three basic ways in which a bipolar junction transistor (BJT) can be used. In each case, one terminal is common to both the input and output signal. All the circuits shown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration
Here the emitter terminal is common to both the input and output signal. The arrangement is the same for a PNP transistor. Used in this way the transistor has the advantages of a medium input impedance, medium output impedance, high voltage gain and high current gain

Common Base Configuration
Here the base is the common terminal. Used frequently for RF applications, this stage has the following properties. Low input impedance, high output impedance, unity (or less) current gain and high voltage gain.

Common Collector Configuration
This last configuration is also more commonly known as the emitter follower. This is because the input signal applied at the base is "followed" quite closely at the emitter with a voltage gain close to unity. The properties are a high input impedance, a very low output impedance, a unity (or less) voltage gain and a high current gain. This circuit is also used extensively as a "buffer" converting impedances or for feeding or driving long cables or low impedance loads.
In both the the common base and emitter follower configurations, the input and output signals are in phase, but with the common emitter configuration only, the input and output signals are phase inverted, a positive input resulting in a negative output and vice versa. This is also known as phase displacement.