Aircraft instruments fall into three basic categories according to their mode of operation: 

1. Pressure Instruments — Altimeter, Airspeed Indicator, Vertical Speed Indicator

2. Gyroscopic Instruments —Turn Coordinator, Attitude Indicator, Heading Indicator

3. Magnetic Instruments — Magnetic Compass

Instruments can also be classified in three ways according to their function:

1. Control Instruments (detect immediate attitude and power changes) — Attitude Indicator, Manifold Pressure Gauge, Tachometer, Fuel Flow Gauge

2. Performance Instruments (reflect a change in velocity or 3-dimensional direction) — Altimeter,   Airspeed Indicator, Vertical Speed Indicator, Heading Indicator, Slip/Skid Indicator

3. Navigation Instruments (indicate the position of the aircraft relative to a selected navigation  facility or fix) — Compass, VOR, GPS, Moving  Map Display, Localizer, Glide Slope Indicators

The Pitot/Static System provides information to the Airspeed Indicator, Altimeter, and Vertical Speed Indicator, whereas gyroscopes power the Attitude Indicator, Heading Indicator, and Turn Coordinator. Gyroscopes have two important characteristics: Rigidity (prevents the axis of rotation from tilting as the earth rotates); and Precession (an applied force will be felt not at the point of application, but 90 degrees from that point in the direction of rotation—the Turn Coordinator uses this principle).