The FBI is also known as the Federal Bureau of Investigation. Based on the name, you can already guess that they are in charge of the investigation of different cases that may occur. They may be assigned to research and know details about cases that are considered to be hard to solve. They may be assigned with cold cases that seem to have no clue.
The police will be in charge of responding whenever there are some people who need immediate help or are not following the rules. For example, someone who is overspeeding may be stopped by the police to prevent the person from getting into an accident. The FBI and the police have different jobs to do that may sometimes seem similar to each other.
The FBI is also known as the Federal Bureau of Investigation. This is known to be the law enforcement agency that is in charge of gathering details about people or organizations that may cause some threats to the United States. They will make sure that all of the threats that may cause issues with the government will be eliminated. The police are different.
They are in charge of making sure that the civilians in their current area are following law and order. If there are some things that you are experiencing in your area, you need to call the police and not the FBI to report your issues.