A bipolar transistor is an amplifying or switching device constructed of doped semiconductor. A bipolar transistor has a sanswich, either NPN or PNP. The center is called the "base" of the transistor. By varying the voltage between the base, and one terminal, one can vary the current flow threough the other terminal of the transistor, causing amplification of the signal at that terminal.
Conceptually, one can understand a bipolar transistor as two diodes placed back to back. The change in voltage at the base electrostatically attracts holes or electrons to jump the junction gap of one diode, while the other diode permits the difference in voltage to exist.
Bipolar transistors have different regions of amplification. In the "linear" range of control voltages, current flow is proportional to voltage, and outside this region, they follow a complex curve, or operate as switches.