The variable type of capacitors can vary the capacitance by changing the distance between the plates or the effective area of the capacitor.
These capacitors use air as the dielectric medium. The distance between the plates can be varied to change the capacitance. The capacitance values offered are high and can be used with high voltages. These are used for high frequency operations in communication systems.
These capacitors have glass or ceramic encapsulation and vacuum as the dielectric. Their complex construction makes it very expensive. Theoretically, it has less losses and are used in RF applications.