Ans:
There are a few key differences between a police complaint and an FIR.
An FIR is a formal document filed with the police, while a  complaint to police can be filed with the police or with another authority, such as a court.
An FIR must contain the names of the accused and the complainant and other essential details about the incident, while a police complaint can be made anonymously.
An FIR can only be filed concerning cognisable offences, while a police complaint can be filed for any offence.
Finally, an FIR must be filed in person, while a complaint to police can be made online or over the phone.