The diffference most people get confused with is gauge pressure and absolute pressure.

Pressure measurements may be expressed relative to various zero references. Absolute pressure of a fluid is referenced against a perfect vacuum. Gauge pressure is referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Atmospheric pressure is typically about 1 bar at sea level, but is variable with altitude and weather. A standard value of atmospheric pressure has been defined to be 101.325 kPa. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For gauge pressures several times larger than atmospheric pressure, this variation is small as a percentage of reading and may be ignored.

Examples of absolute pressure measurements include barometric pressure, altimeters, and the Manifold Absolute Pressure (MAP) sensor used in the engine control systems of modern fuel-injected cars. Examples of gauge pressure measurements include tyre-pressure gauges and sphygmomanometer.