Bipolar transistors are called bipolar because the flow of electrons only takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or visa-versa). In other words, two types of charge carriers -- electrons and holes - are responsible for the generation of current.
A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.
Some of the basic properties exhibited by a Bipolar Transistor are same as diode. To understand working of Bipolar Transistor, let us first consider NPN variety.
In the case of P-type (filling) layer conduction can take place by the movement of the free holes in the valence band.