Question

difference b/w bypass and coupling capacitors

Answer 1

A coupling capacitor is used to block dc whilst allowing ac part through... depending on the value chosen depends on what frequencies you allow.
a bypass capacitor is used to remove noise or other unwanted signals or frequencies from a dc or low frequency signal. .

Answer 2

It is the same capacitor being used for two different purposes... one ( coupling capacitor) is used to couple an output of a say transistor to input of another transistor...second (bypass capacitor) is used to bypass the ac current through a resistor by connecting a capacitor across it to the ground , hence moving all of the ac to the ground without loosing any power! this link explains it in one line (4 th line)
http://en.wikipedia.org/wiki/Decoupling_%28electronics%29