Sometimes a person is arrested for a crime they did not commit. In the nightmare scenario, they are charged and maybe even convicted. But often the prosecutor declines to prosecute or, if charged, the charges are dismissed or the person is acquitted.
For example, say you are at a party and someone starts a fight. You are in the fray but only as a spectator. You do not throw a single punch or otherwise harm anyone, but some of the participants are injured. The police arrive and you find yourself arrested for aggravated assault, which is a serious charge. The subsequent police investigation exonerates you: plenty of witnesses tell the investigators that you were not one of the participants but only standing on the sidelines. The district attorney recognizes that there is no reason to pursue charges against you. All you suffered is the arrest and nothing else; you are in the clear, right?
Not exactly. That arrest is now on your otherwise pristine California Department of Justice criminal history record (commonly called a “rap sheet”). In the era of instant information, having an arrest show on your criminal record means that current and future employers may find out about it, your school may learn of it, even your landlord may know. Furthermore, if you have a job that requires a state or federal license or clearance, the arrest may affect your job or future application for such a job.