A professional is someone who does something as a profession, or that they receive pay for. The term professional can also indicate that someone has great skill in a craft or activity, or that something demonstrates such skill. To conduct oneself as a professional (professional behavior) would indicate that the person's actions are in accordance with specific rules, written or unwritten, pertaining to the standards of a profession.
The opposite of professional is amateur. In many cases someone can be doing the exact same craft or task, and the only difference between a professional and an amateur is that one is paid. An example would be athletes in some sports.
In many areas there is a barrier to entry that must be passed before someone can become a professional. Such barriers include academic degrees, certifications, licenses or completion of an apprenticeship. Professions with such barriers include doctor, engineer, lawyer, plumber, and electrician.