Pressure is another word for force and is applied to a specific area. A simple example of pressure may be seen by holding a knife to a piece of fruit. You put pressure to actually cut the fruit. This is a type of pressure and in total you have four. All of them are explained here. Maybe you will even recognise them.
You can kind of compare absolute pressure to a full vacuum. The absolute pressure measurement is required for the ideal gas law in the same sense that temperature must be represented by its absolute unit, and that is the Kelvin. Absolute pressure can be measured with an electronic diaphragm sensor, where the measured pressure is applied on one side of the diaphragm, while the other side of the diaphragm is exposed to a permanently sealed vacuum.
Relative pressure is also known as overpressure. Relative pressure is the pressure difference between the absolute pressure of the atmosphere and another absolute pressure value. It often gets confusing. The difference between them is that Absolute barometric pressure is the actual pressure exerted by air at a given location. Relative pressure is what the pressure would be at sea level given the actual absolute pressure at a location above or below sea level. Typical relative pressure applications are air compressors or process control pumps, these can be controlled with pressure control systems.
It actually explains itself. It is pressure that is trapped within the back of the diaphragm, similarly to absolute. But it does not use a vacuum like with absolute pressure, but a predetermined reference point, thus venting on the backside is not needed for pressure to flow in or out. A simple but very accurate example would be an ear infection. It is caused when the vent tube from your ear to your sinuses becomes clogged. This seals fluid inside your inner ear cavity, which makes changing elevations really painful.
Also kind of obvious but with differential pressure, the difference between two pressure points is measured. Often the difference in pressure is caused by changes in air density, and air density is also related to temperature. Warm air is less dense than cooler air because the gas molecules in warm air have higher speed and are farther apart than in cooler air. You can see this in for example filter monitoring or flow rate measurements. This type of pressure also has different industries and pressure sensors.
Now you know what the four types of pressure are. If you look closely you can see almost everything all around you in your daily life. It doesn’t have to be that difficult. And hopefully it is somewhat easier to understand now.