Capacitor

Capacitor stores charge.  It composed of 2 metallic plates with a insulation gap (ceramic, plastic, air etc) in between.  When a voltage is applied to the capacitor, it takes a finite time to charge up.  The duration of charging depends on the capacitance value of the capacitor.  The capacitor provides a DC voltage to the load when discharging.  Again, the discharge will take a period of time.

When used with AC, capacitor behave like a resistor and the opposition of current flow is called capacitive reactance.  Unlike a resistor, capacitor resistance varies depending on the frequency of the AC and the capacitance values.  In other words, the capacitor will only passes voltage of a certain frequency range and it attenuate the frequency outside the range rapidly.

Capacity can also use to block DC and allow AC to pass through.