A criminal is a person who has committed crimes. Criminals can be caught by the police, tried by the courts and if convicted receive punishment such as a term in prison or death.
Matters related to criminal behavior in society are studied in the field of sociology on the sub-field of criminology, and a person who studies this is called a criminologist. Psychological assessment of criminals is studied by psychologists.
The term "criminal" is sometimes used as a derogatory accusation of a person when viewing an act attributed to him/her as dangerous or destructive towards other people, or property. However, legally, in almost all jurisdictions, it is acceptable that a person is not guilty until convicted, thus may not be called a "criminal" until proven guilty.
A criminal has a criminal record, detailing the allegations and findings against them.