This applet shows a simple circuit involving a capacitor, which is a device that stores charge. As current flows into the capacitor, the voltage across the capacitor increases. As its voltage approaches the source voltage (the 5V voltage source shown on the left), the current flowing into the capacitor decreases.
Click on the switch to discharge the capacitor, and then click on it again to charge it again.
The scope below the circuit shows the voltage across the capacitor in green, and the current in yellow.