Everyday pressure measurements, such as for your car tyre pressure, are usually made relative to the ambient air pressure. In the world of C&I however, measurements are occasionally made relative to some other specific reference, e.g. a vacuum. Therefore it is important to distinguish between these zero references, so the following terms are used:
Absolute Pressure is zero-referenced against a perfect vacuum, using an absolute scale,
Gauge Pressure is zero-referenced against ambient air pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge",
Differential Pressure is the difference in pressure between two points.
The zero reference in use is usually implied by context, and the words are added only when clarification is needed, however it is common to see the letters "g", or "a" added after the unit of measurement to distinguish gauge or absolute respectively e.g. 15 barg.
For most process fluids where the fluid exists in a closed system e.g. within the pipework of a process plant, gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure.
Differential pressures are commonly used in industrial process systems, and indeed form the basis of many instrument systems e.g. level measurement, and flow measurement. Differential pressure gauges (often referred to as DP gauges) have two inlet ports, each connected to one of the volumes whose pressure is to be measured. The DP gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to monitor two separate gauges and calculate the difference in pressures.
You are most likely to come across absolute pressure when measuring vacuums.
It is worth noting that Atmospheric pressure is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For most situations within the oil and gas, and process industires changes in Atmospheric pressure can be ignored.