In general, criminal justice is a system whose purpose is to maintain order in a society. The criminal justice system is mainly concerned with ensuring that laws are not broken. [We could do more to help you if you could make your question somewhat more specific by giving more details about what you need.]
There are three broad parts of the criminal justice system. First, there is law enforcement--the police. These are people who are supposed to prevent crime (by their presence) and who are supposed to work to apprehend those suspected of crimes.
Second, there are the courts. This is the part of the system that decides who is guilty and who is innocent. Those who are found guilty move on to the corrections part of the system. In this part of the system, punishment is meted out to those who have been found guilty.
These three broad areas make up the criminal justice system. They work together in an attempt to minimize the amount of law-breaking that occurs in a society.