How does a transistor works as an amplifier? I want to know the reason of this amplification.Why does a small change in base current causes a large change in collector current?

Hi,
Here is a link with some good explanations:

http://www.saburchill.com/tech/electronics/elect0121.html

It also contains more links for explanations.
Best wishes.

simple enough. A transistor converts d.c. power from the battery into the signal waveform. That is it converts d.c. into variable voltage(current) according to input signal.