This chapter deals with a very important active component used in electronics. Since the implementation of the BJT (Bipolar Junction Transistor), sizes of electronic devices and computers could be shrinked enormously. e.g. In a modern CPU chip a few billion transistors are integrated. Basically a transistor is a (base)current amplifier. It can be used as a switch, a current amplifier , a voltage amplifier and many more things.
A PDF ON TRANSISTORS:Transistor
See also here.