When people ask What is Criminal Justice they are actually referring to 3 individual parts that make up the Criminal Justice System.
The first is part is law enforcement including local, state, and federal. Law enforcement is the first element of What is Criminal Justice as it prevents, investigates, and assists in prosecuting those accused of a crimes.
Law Enforcement would be the first contact someone would have with the criminal justice system. Law enforcement would be the first responders to the scene of the crime and would interact with victims, witnesses, and defendants.
The 2nd part is the Court System. The Court System is in charge determining the facts, finding defendants innocent or guilty, and deciding upon a sentence.
Courts would include judges, attorneys, as well as investigators working for the defense.
The 3rd part Probation Officer is corrections which is made of of jails, prisons, probation and parole. These would include correctional officers, prison guards, as well as probation and parole officers.
If a defendant is found guilty they may receive a sentence which includes incarceration and/or supervised release.