Refer to figure.
Static Pressure is the pressure exerted on bodies in the atmosphere, as a result of the atmosphere's Weight. It acts everywhere on the aircraft. The static pressure decreases with altitude. Static pressure is measured using static ports aligned in such a way that they don't also measure some dynamic pressure.

Dynamic pressure is the pressure experienced due to movement through the air. It is related to the speed at which the aircraft moves through the air:

P_Dynamic = ½ρV²

Dynamic pressure can be measured using a pitot tube. The pitot tube is aligned with the direction of flight. Air that flows into the tube comes to a complete stop, and the pressure is measured. This will be the total pressure which is the combination of static and dynamic pressure. In order to measure the dynamic pressure, the static pressure has to also be measured and subtracted from the total pressure.

Because dynamic pressure is related to speed, the amount of dynamic pressure that is measured can be converted to a speed in knots and shown on the airspeed indicator. The ASI requires both static and dynamic pressure to display the correct reading.

Other instruments, such as the altimeter and VSI, only require static pressure to work.

The Attitude Indicator does not require either static or dynamic pressure, but on light aircraft usually requires vacuum pressure, provided by an engine-driven vacuum pump. More modern aircraft are starting to use systems that use lasers, instead of rotating vacuum-driven gyroscopes.