In many ways, a private investigator is like a police officer, sheriff's deputy, or a police detective (all referred to as "peace officers"). As such, the primary function of a P.I. is to undertake an investigation. And like peace officers, a private investigator may investigate a whole range of situations. 

There are several differences, however, between a private investigator and a peace officer. First, a private investigator is a person who conducts investigations as a private citizen, possibly as a member of a private agency, or as an adjunct member of a law firm.  The private investigator is paid by a private citizen, a business, a law firm, or the agency for which the P.I. works. A peace officer is a public servant employed by a government agency, like the municipal police department, the state police, the county sheriff, etc. Tax dollars pay for the peace officer. 

Second, a peace officer investigates crimes that have already been committed, so in criminal matters, a peace officer's job is reactive, meaning they react to the consequences of a crime already committed. It is at that time that the peace officer investigates the circumstances of the crime from the state's perspective in order to identify and prosecute the offender (perpetrator). On the other had, the private investigator usually works for the defense attorney or public defender to represent the interests of the person who's been accused of having committed the crime. In some cases, a private investigator is called in to prevent a crime from being committed, like when the P.I. conducts surveillance on a former spouse to prove an intent to harm; or if the P.I. is engaged to remove property or locate a person or thing before a crime is committed. 

Police do not investigate civil matters -- only criminal. In civil matters, a private investigator's role could be viewed as either reactive or proactive. When investigating a case where a wrong has already been committed, the PI is reactive, gathering data to support his or her client's position. When gathering information before a wrong has been committed (doing background checks, searching for assets, finding missing people, investigating many family situations), the PI acts proactively to prevent a wrong from being committed. A good example might be a security guard who acts proactively to prevent shoplifting or the wrongful entry onto private property merely by their presence.

Private investigators are also called private detectives, PIs, or private eyes. More recently, however, private investigators have used the professional title, ‘professional investigator’ or even 'legal investigator'. Primarily, however, the change in title is an effort to negate the somewhat ‘seedy’ image that has been attributed to private investigators... courtesy of Hollywood and many fiction writers.