Can anyone explain the Ignition system of piston engine in detail?
 

I get confused everytime about the magneto circuit.
What happens when the ignition switch is placed in the ON position?
What is impulse starter?

Let me google that for you

It only takes seconds....

The ignition switch allows the magneto to do its bit towards the 'bang' part of the 'suck, squeeze, bang, blow' cycle. The impulse starter - well you guessed it - helps start the sucker in the first place. What may be confusing you is that it is not a separate starter motor at all, but a part of one (or sometimes both) the magnetos. Starting assist may also sometimes be referred to as a 'shower of sparks' ignition.

The really, really important thing to know about the ignition switch on a magneto-spark engine is that OFF may not really be OFF. If the grounding (or earth) is not secure the magneto will still be 'live' after you shut the engine down on the mixture. Which is why you do a dead-cut check prior to shutdown (I hope).

As Saigon says, all of this in much better detail can be found by googling. Google will give it to you as scientifically, or as basically, as you want.