The definition of a criminal may differ from how society sees it and how the law perceives it. While having been cheated on by that unkindly ex-husband might be a violation of moral laws, criminal law is a whole different matter. In short, a criminal is "one who is found guilty of a crime." While that may appear as a pretty clear-cut definition, its meaning goes much deeper.
The law is separated into four different types of crimes. There are petty crimes, misdemeanors, felonies and wobblers. Issuing a written citation, such as a traffic violation, generally falls under the category of a petty crime. A misdemeanor is more serious than a petty crime, but the violator retains their rights in respect to joining the armed forces and being able to vote. If jail time is handed down, time is spent in the county jail rather than a prison. In the event of a first offense the sentence could be probation.
When confronted with a wobbler you're sitting on the edge. A wobbler can go either way and the perpetrator could be charged with either a misdemeanor or a felony. Prosecutors find this to be handy leverage when it comes time to plea bargain.
A felony is the most serious crime that one can commit and they vary in degree. A felony is generally defined as any crime that is punishable by more than one year in a penal institution. Once convicted of a felony many of your rights are taken away. You can no longer join the military, serve on a jury and can lose your right to vote. Additionally, it can effect your career choices. A convicted felon cannot become a member of the bar or be certified as a teacher in many states.
While a criminal may be someone who acts outside the norms of society, the law distinctly states that a criminal is one who is "convicted" of a crime. In general, that conviction would be for a felony. After the initial arrest, the suspect has the right to post bail until the court can hear their case. In some instances, such as a homicide, that right is lost. It is every person's right to be presumed innocent until proven guilty, and the burden of proof lies with the prosecution. If a court of law finds them guilty, their innocence no longer plays a role.
What a criminal looks like is a misconception among society. The man that you see mowing his lawn from your front window could appear as an upstanding member of society and be a registered sex offender. The man that you saw at the grocery store covered in tattoos and sporting a multitude of body piercings could well be a minister. Criminals have no particular appearance, and seldom can you pick them out of a crowd. They can come across as sociable and highly intelligent individuals, and oftentimes they are.
While there is no way to determine every person's criminal past, there is a national registry online where anyone can check for sex offenders in their area (http://www.familywatchdog.us/). Arizona has implemented a program where the public can check for warrants on individuals and, hopefully, other states will follow suit.