PSI and PSIG-- Pounds per Square Inch, and Pounds per Square Inch Guage. Both are units of measure to indicate the pressure on a surface. Technically, PSI starts at zero relative to a true vacuum (such as that in space). PSIG is the technically correct term to use when referring to a pressure gauge which has been calibrated to read zero at sea level (which is just about every guage you've probably ever seen).

At sea level, the earth's atmoshpere actually exerts a force of 14.7 psi on all surfaces. We don't feel this pressure, because we have always lived under these conditions and we're used to it. If you were to calibrate a pressure guage to read zero while on the moon, then travel back to sea level on earth, your guage would read 14.7 psi.

When you read 30 psig on a tire guage, that measurement relative to a zero pressure environment (such as that in space) is in fact 30 psig plus the 14.7 psi of atmospheric pressure. The true psi relatve to a vacuum is 44.7 psi. However, for virtually every purpose, most of us are concerned with the differential of the pressure to the atmosmosphere. Therefore, guages are set to zero, and show us the additional pressure above the 14.7 sea level atmospheric pressure.

Whenever you read any guage such as for tire pressure, the technically correct unit of measurement is psig. However, the lay person has gotten in the habit of using the lazy form of psi when psig should be used.