please enlighten me with a very precise note on Coulomb

1 coulomb is that quantity of electric charge which exerts a force of 9 x 10⁹ N on an equal charge placed at a distance of 1 m from it.

It is the fundamental unit of electric charge.

denoted by C.

It is equal to the charge transported in one second across a conductor in which there is a constant current of one ampere. 

Charge = Current x time 

Thus, 

1 Coulomb = 1 Ampere x 1 second 

Thus, 

Coulomb can also be written as Ampere - second.