Criminal justice refers to legal policy, policing, courts and the corrections process -- the universal aspects of law enforcement. Classes in criminal justice focus on how the system works, how criminals affect the system and how said system should be administered.
Differing from criminal justice, criminology focuses on the behavior and motivation of criminals, as well as society's responses to crime. Those who study criminology should expect to take classes that focus on the interactions of law-making and law-breaking, as well as the reactions of society to both. This field draws heavily on sociology, so classes may overlap with sociological studies.
Many criminal justice and criminology programs interchange courses frequently, not completely differentiating between the two subjects. Regardless of your area of focus, as a student you should be prepared to study both disciplines. For example, a criminal justice study of corrections and the court system could go into how those systems affect criminals psychologically, therefore venturing into the realm of criminology.
Those who go into the field of law enforcement, with either a degree in criminal justice or criminology, tend to go into the following careers: police officer, detective, criminal investigator, federal agent, corrections officer, corrections counselor, lawyer, academic researcher, juvenile treatment worker, community correctional worker, probation officer, parole officer, private investigator and private security, among others. Those who specialize in criminology are better suited for the careers that focus on the offenders and their states of mind, like detectives and criminal investigators.