You are a criminal only AFTER you have been convicted of a crime.
Criminal law involves prosecution by the government of a person for an act that has been classified as a crime. Civil cases, on the other hand, involve individuals and organizations seeking to resolve legal disputes. In a criminal case the state, through a prosecutor, initiates the suit, while in a civil case the victim brings the suit. Persons convicted of a crime may be incarcerated, fined, or both. However, persons found liable in a civil case may only have to give up property or pay money, but are not incarcerated.
A "crime" is any act or omission (of an act) in violation of a public law forbidding or commanding it. Though there are some common law crimes, most crimes in the United States are established by local, state, and federal governments. Criminal laws vary significantly from state to state. There is, however, a Model Penal Code (MPC) which serves as a good starting place to gain an understanding of the basic structure of criminal liability.
Crimes include both felonies (more serious offenses -- like murder or rape) and misdemeanors (less serious offenses -- like petty theft or jaywalking). Felonies are usually crimes punishable by imprisonment of a year or more, while misdemeanors are crimes punishable by less than a year. However, no act is a crime if it has not been previously established as such either by statute or common law. Recently, the list of Federal crimes, dealing with activities extending beyond state boundaries or having special impact on federal operations, has grown. See Title 18.
All statutes describing criminal behavior can be broken down into their various elements. Most crimes (with the exception of strict-liability crimes) consist of two elements: an act, or "actus reus," and a mental state, or "mens rea." Prosecutors have to prove each and every element of the crime to yield a conviction. Furthermore, the prosecutor must persuade the jury or judge "beyond a reasonable doubt" of every fact necessary to constitute the crime charged. In civil cases, the plaintiff needs to show a defendant is liable only by a "preponderance of the evidence," or more than 50%.