There is a difference in the law between civil and criminal cases.
The main differences are that imprisonment is not a remedy in a civil case and the burden of proof is greater in a criminal case.
In a civil case. the plaintiff need only prove his/her case by a preponderance of the evidence. Such a case can be filed even though the police and the State's Attorney feel there is insufficient evidence to prove a criminal case beyond a reasonable doubt.
Examples of cases that cannot be charged by the State's Attorney include those situations in which someone owes you money and has refused to pay you back. To most people, this behavior is theft and the person ought to be charged with a crime. However, the law requires that the State prove that the person had the criminal intent to deprive at the time the person took possession of the money.
In most cases, this element is missing. Without evidence of criminal intent, the State's Attorney would be unable to prove a criminal case beyond a reasonable doubt. In such a case, a small claims case can be filed in order to resolve the matter.