What is the difference between a Criminal case ans a Civil cas?

In a criminal case, the government prosecutes and seeks to punish a person (or entity) for a crime he or she committed which (although only one person may be the victim) is considered to be an offense against society as a whole, and the case generally does not involve a question of recovery by the victim; generally, the victim has to bring a civil case in order to recover damages.  In order to convict the defendant, the prosecution has the burden of proving its case beyond a reasonable doubt. A civil case is a private dispute between individuals or entities, and involves private wrongs committed by one against the other.  The idea, usually, is not to punish (although punitive damages are sometimes awarded for some types of claims) but to make the injured party whole by awarding him/her/it damages, and/or equitable relief (any relief other than money, for example an injunction requiring someone to stop using a name which infringes another's trademark, is equitable).  The party claiming to have been wronged generally has the burden of proving his/her/its claim by a preponderance of the evidence, not beyond a reasonably doubt. Criminal and civil cases are not mutually exclusive; the same facts can give rise to both.  If you recall, OJ Simpson was acquitted in the criminal case brought against  him for allegedly murdering his ex-wife and Mr. Goldman, but was found liable in the civil suit for wrongful death brought be Mr. Goldman's parents.  He did not go to jail (for that offense), but was required to pay a large amount of damages for wrongful death.  In the criminal case, OJ could not be convicted unless proven guilty beyond a reasonably doubt; in the civil case, the Goldmans only had to prove their wrongful death claim by a preponderance of the evidence. The claim you describe above could give rise to both a criminal complaint for theft, and a civil claim for conversion.  That doesn't mean that either one would succeed; I don't know what your niece's side of the story is.  The local police and prosecutor may be declining to prosecute because they think the case is too weak to give them a good shot to prove your niece's guilt beyond a reasonable doubt.  They may think (I"m just speculating) that this is more of a family dispute than a criminal matter in which the government should get involved.