Charging a Capacitor:

The voltage across the capacitor is not instantaneously 
equal to that of the voltage across the battery when the 
switch is closed. The voltage on the capacitor builds up as 
more and more charges flows onto the capacitor until the 
battery is no longer able to "push" any more charge onto 
the capacitor, at which point the capacitor becomes fully 
charged.
The initial flow of charges from the battery to the 
capacitor means that there is a current flowing through the 
system until the capacitor is charged. This current flow 
decays exponentially from some initial value to zero.
DisCharging a Capacitor:
Switch remains open and voltage across capaciotr decreses 
untill it reaches zero.

the capacitor is charge when the current flow through the
ckt and its discharge whenever appllied voltage is stopped.

Capacitor is made of say two plates seperated by dielctic 
material. When volage is apllied the ions on the plate are 
zero so initially more current will flow like in short 
circuit.
As time passes the charges(+ve and -ve) accumalate on 
plates over a time and it opposes current flow. Now we say 
capacitor charged fully. Now the time taken to charge fully 
is called time constant of a capacitor. This time can be 
varied by connecting a resistor in series with capacitor. 
When high resisor is connected voltage drop across 
capacitor is reduced so it take long time to charge fully.
Vica-versa for low resitor. The relation is exponetial 
charging and discharging. Discharging process is also 
similar way.