What is the difference between high-pressure systems and low-pressure systems?

Answer 1

Low pressure systems are made up of low airmass and pressure. High air pressure systems are made up of high airmass and pressure.
Standard atmospheric pressure is 14.7 psi, 760 mm Hg, 101 kiolopascals, or 13.33 mb. All these units indicate the pressure of the atmosphere at sea level. High or low atmospheric pressure indicates that the pressure is higher or lower than this standard. Aside from weather related highs and lows, the pressure is lower the further up you go and higher the further below sea level (like ain mine shaft) you are.

Weather related air pressure is primarily related to air temperature (cold air is heavier and warm air lighter)

Answer 2

Low pressure systems are made up of low airmass and pressure. High air pressure systems are made up of high airmass and pressure.
Standard atmospheric pressure is 14.7 psi, 760 mm Hg, 101 kiolopascals, or 13.33 mb. All these units indicate the pressure of the atmosphere at sea level. High or low atmospheric pressure indicates that the pressure is higher or lower than this standard. Aside from weather related highs and lows, the pressure is lower the further up you go and higher the further below sea level (like ain mine shaft) you are.

Weather related air pressure is primarily related to air temperature (cold air is heavier and warm air lighter)