A professional does something as a profession, or receives payment for some activity. The adjective "professional" can indicate that someone has great skill in a craft or activity, or that something demonstrates such skill. To conduct oneself as a professional (exhibiting "professional behavior") would indicate that the person's actions remain in accordance with specific rules, written or unwritten, pertaining to the standards of a profession.
The opposite of "professional" is "amateur" (disparagingly: "rank amateur"). In many cases someone can perform the exact same craft or task, and the only difference between a professional and an amateur consists in the payment of the one but not the other.
Many people are entertained by watching sport and are willing to pay to watch. This allows some contestants to make a living as professionals by getting paid to play professional sports. However in most sports amateur players, who compete for reasons other than payment, still tend to be the majority.
In many areas a person must overcome a barrier before gaining recognition as a professional. Such barriers include academic degrees, certifications, or licenses. Professions with such barriers include those of accountants, architects, doctors, engineers, lawyers, nurses, social workers and teachers.
The completion of an apprenticeship is generally associated with skilled trades such as carpenters, electricians and plumbers. Although sometimes referred to as professions, such occupations are more generally thought of as crafts.
Compare the usage of "occupation", which does not imply the exclusivity and special nature of the "professional".