Wikipedia on Answers.com:

 

Professional

A professional is a person who is paid to undertake a specialised set of tasks and to complete them for a fee. The traditional professions were doctors, lawyers, clergymen, and commissioned military officers. Today, the term is applied to estate agents, surveyors, environmental scientists, forensic scientists, educators, and many more. The term is also used in sports to differentiate amateur players from those who are paid â€" hence "professional footballer" and "professional golfer".

In some cultures, the term is used as shorthand to describe a particular social stratum of well educated, mostly salaried workers, who enjoy considerable work autonomy, a comfortable salary, and are commonly engaged in creative and intellectually challenging work. Less technically, it may also refer to a person having impressive competence in a particular activity.  ;D

Chris.