Altimeter: The altimeter measures the altitude of the plane. This instrument uses the fact that as the plane gains altitude the air preasure decreases. The instrument can sense th air preassure and the change in the preassure. So as the plane rises the preasure decreases and the altimeter senses it and shows it.
Vertical Speed Indicator: The vertical speed indicator shows weather the plane is moving up or down. It does this through measuring the change in air preassure.
Airspeed Indicator: The airspeed indicator shows the speed which the plane is moving through the air. This speed is not nessarily the ground speed which can vary with the wind direction and velocity.
Heading Indicator: The heading indicator is used to give the heading of the plane. It is not magnetic it runs on a gyro. Because of this the indicator must be set to the compass before every flight.