A detective is an investigator, usually a member of a law enforcement agency. They often collect information to solve crimes by talking to witnesses and informants, collecting physical evidence, or searching records in databases. This leads them to arrest criminals and enable them to be convicted in court.
When you think of criminals, your first thought might be of someone awful like a murderer. But this word is a lot broader — Anyone who breaks the law is technically a criminal, even if the crime is just not paying a speeding ticket. You can also talk about criminal activities: things that are illegal. Lots of TV shows are about police officers and lawyers trying to catch criminals who are engaged in all kinds of exciting criminal activity. Crime Syndicate - a loose affiliation of gangsters in charge of organized criminal activities