The two principal pressure types are gauge (or gage) and absolute pressure. Vacuum is sometimes considered its own pressure type, although it is a negative gauge pressure. Barometric pressure is also used in discussions; it is the atmosphere’s absolute pressure. Differential pressure is also considered a pressure type, being the difference of two separate pressures.
Let’s have a quick look at these different types.
Gauge pressure is the most commonly used pressure type. With gauge pressure we always compare the pressure we are measuring against the current atmospheric pressure. So, it is the difference between the measured pressure and the current atmospheric pressure, meaning that we are that much above (or below) current atmospheric pressure.
Absolute pressure is the pressure compared to absolute vacuum, so it is the difference of the measured pressure and the absolute vacuum. An absolute vacuum is a state where the vacuum is so deep that there are no air molecules left, so there is no pressure.
Vacuum pressure is a (gauge) pressure that is below current atmospheric pressure. Being a gauge pressure, it is compared against the current atmospheric pressure and is often indicated as negative gauge pressure.
As the name already hints, the differential pressure is a difference of two separate pressures. The value can be positive or negative (or zero) depending on which of the two pressures is higher.
The barometric pressure is the absolute pressure of the current atmospheric pressure at a specific location. The nominal barometric pressure has been agreed to be 101,325 Pa absolute. The barometric pressure is dependent on weather conditions, your location, and your elevation.