anand: yes i20 has a conventional turbocharger (FGT).
emperor: Nice question. Look, the exhaust gas from engine is directed to a set of nozzles (nozzle ring). This drives a turbine, which is connected to a compressor (or blower). This compresses the intake air. This compressed air, when goes to combustion chamber imparts better combustion, hence better efficiency, better power. This way more power is obtained from same engine.
At lower rpms, exhaust gas pressure will be low. So turbine will rotate at lower rpm. So its efficiency will be less. This is reason why we have turbo lag.
To overcome this, variable geometry turbo were developed. In this case, the aperture of nozzles can be changed externally (This is the basic difference between VGT and FGT). When engine is at lower rpm, nozzle aperture is throttled. This causes increase in the pressure of exhaust gases. This causes the turbine to rotate faster------that means better compression of intake air------- that means improved combustion.
So by using VGT, efficiency, power output is maximised for a wider range of rpm.
One practical example. Swift power output is somewhere around 76 bhp. while that of a Manza is 90 bhp. Both have same engine. While swift has FGT, Manza has VGT.