Well, you've got alot of things to learn before worrying about compressor surge, but i'll give you the basics.
The turbo is divided into two parts, the "hot" (exhaust) and "cold" (compressor) side. The 'charge' piping runs between the compressor outlet and the throttle body, containg pressurized air.
The blow off valve (BOV) sits on the charge pipe between the two, discharging unneccesary air when the throttle plate is slammed shut. It is controlled by a vacuuming line, and when there is a change in vacuum pressure, it is pulled open, venting the air to aptomosphere.
Also, on (most all turbocharagers, with the exception of some race only applications) there will be a wastegate. The wastegate regulated boost pressure by opening a second channel through which exhaust gas may flow, reducing the pressure on the turbine wheel (thats the wheel in the exhaust side that powers everything) so that the turbo cannot spin faster.
Wastegates may be mounted internally, in the turbocharger, or externally, on the exhaust manifold. No matter where it is mounted, though, they all function in the same way: boosted air pressure pushes against a spring until it opens it to a certain point.
Boost controllers change when a wastegate opens and closes by modifying the signal (boost pressure) that the wastegate sees. Boost controllers (manual ones, anyways) generally function by the tightening of a screw against a spring to increase the pressure required to open it. Essentially the more pressure required, the more "boost" there is before the spring is compressed and air is allowed to reach the wastegate, opening it.
There are also "Diverter Valves" (DV) they function in the exact same manner as a BOV, excpet that they are used on cars which utilize a Mass Air Flow style air meter. The reason for this can be summarized as below:
Air -----> flow meter ---->turbo----(past BOV) ---->throttle body
In this situation, if the BOV discharges when throttle is dropped, the air vented to aptmosphere has already been metered by the ECU, and the computer still flows fuel for lost air, resulting in a rich condition. (So you basically get lots of nasty black smoke and bad gas mileage)
In systems like this a "Diverter Valve" is used. these valve instead of venting the air to aptomosphere recirculate it, after the airflow meter, but before the turbo.
Anything else I can help with?
