What Is a Criminal?

I was talking with an acquaintance the other day who is a police captain in a near-by county. He was relating a frustration from work. His people were conducting prostitution stings based on Craig's List ads. The johns who were busted included some well-respected members of the community and he received flack from above for wasting the force's time and resources busting people who were not really criminals. His contention was that a criminal is anyone who commits a crime and these people did, making them every bit a criminal. Surely, there is some element of class and race here, but it seems a fair question, what do we mean by "criminal"?