The Full form of IG is Inspector-General of Police. In several nations, IG is a senior police officer in the police force or police service. The rank usually refers to the head of a large regional command within a police service, and in many countries refers to the senior most officer of the entire national police. When an IPS officer completes 19 years of service, he is entitled to become an IG of Police.
In another way, one can write state Civil Services Examination [also called as Group I examination] conducted by the SPSC (state public service commission), get a good rank in it and join as a Deputy Superintendent of Police [DSP] in his home state. The IG, who is appointed by the President subject to Senate confirmation, reports to the Attorney General. The Office of Inspector General investigates allegations or complaints of misconduct or wrongdoing by employees or contractors that involve or give rise to fraud, waste or abuse within the programs or operations of the FCC.
The DGP is the senior most officers in the entire police department who heads the state police force. An IGP is one who is also one of the senior most officers in the police force who eventually on promotion will become an ADGP and finally DGP.