The stock boost gauges, on both of my WRX & STi, measure in MPa. "MPa" is Megapascals
(The pascal unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher noted for his experiments with a barometer. The name pascal was adopted for the SI unit newton per square metre by the 14th CGPM in 1971.)
The conversion of Mpa into psi is basically: .10 MPa = 14.5 psi (Actually it's more like 14.503770000000001 psi, but 14.5 is close enough.). Or you could always just go to one of the many conversion calculators on the web like this one.
As far as what it is going on when the gauge is moving... That's a little more then I feel like typing at the moment but read here.