Air pressure is measured in the manifold or air intake after the turbo by a Manifold Absolute Pressure Sensor (or MAP sensor).
1, On the these I believe it's down the back under the engine cover/airbox, in the top of the manifold.
Seen here in the last few pics
http://www.fiatlinea.org/fiat_linea_bakim/map_sensor_temizligi-t3681.0.html
2, It detects what air pressure is being fed into the engine and along with the Mass Air Flow sensor (MAF sensor) the ECU can work out the available oxygen, then works out the correct fuel strategy.
3, It's a sensor, so it will receive a signal voltage (often 5v), alters it due to whatever is happening and returns this altered voltage back to the ECU.
It works like a balloon with a little air in it, put the balloon in a vacuum and it expands (though the amount of air inside stays the same), put it under pressure and it contracts (again, the amount of air inside stays the same).
4, As already stated, it's signal, along with other sensor readings help the ECU work out fueling stategies, so will influence the injector timings and bandwidths, basically the more oxygen available, the more fuel it can throw in.
As boost pressure is not constant, it will also influence turbo control in some applications with modern Variable Vane Turbos (older VGT's and wastegate turbos tend to control themselves via a vacuum loop), so as manifold pressure rises, the vanes will contract and limit boost.
Engines can only handle a certain amount of boost reliably, so in general the faster the engine/exhaust gases, the higher the boost, the more the vanes/wastegate will contract and vice versa.
If the ECU detects too high a MAP signal, it will protect the engine by tripping limp mode and limit power/boost.
5, Again boost is related to engine/exhaust gas speed but vanes and wastegates try to control boost to a useable level.
Generally they will boost to around 1 bar above atmospheric pressure (which is near enough 1 bar at sea level) but it's often reported by diagnositic tools in kilopascals (kPa's).