What's the Difference Between Gauge and Absolute Pressure

Absolute Pressure
The absolute pressure - pa - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. All calculation involving the gas laws requires pressure (and temperature) to be in absolute units.
Gauge Pressure
A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure.Therefore Gauge pressure is measured from atmospheric and absolute is measured from 0 (as all absolute scales are measured from). They both use the same scale for measuring.
Standard atmosphere at sea level is 14.7lb/in^2 of pressure absolute and 0 pressure gauge. If you had 100 psi gauge pressure you'd have 14.7+100psi absolute.Atmospheric Pressure
Atmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure varies with temperature and altitude above sea level.