turbochargers draw more air through vaccum into engine,induce more fuel in the engine to get than maximum power frm a certain sized engine.
keeping increasing the size of engine is not always possible as it wud cost more,nd wud b havier so we use trubo/superchargers instead
here is a bit how turbochargers work
the exhaust gasses r drawn into a special part of engine where they helf run the turbines to drive the T.C.then,as they r driven,the compress the air,cool it by specially designed radiators,nd then push them into the engine.increasing engine compression.more fuel is also drawn into the engine to burn as more air is availabe hence lot more power is generated.they can increase upto 40%,the performance of an engine.
the usually work at higher rpms than the engine...may b 50,000 or 60,000rpm i guess
try this site for detailed info