A professional works to receive payment for an activity (as a profession), which usually requires expertise and carries with it socially significant mores and folkways. That is to say, behaving professionally would indicate that the person's actions remain in accordance with specific rules, written or unwritten, pertaining to behavior, dress, speech, etc. By extension, the adjective professional can indicate that someone has great expertise or skill in a craft or activity.
In narrow usage, not all expertise is considered a profession. Although sometimes referred to as professions, such occupations like construction work are more generally thought of as crafts. The completion of an apprenticeship is generally associated with skilled labor or trades such as carpenters, electricians, plumbers, and other similar occupations.
The opposite of professional is amateur or disparagingly rank amateur. Sometimes an amateur can perform as well or better than an professional, but this is tends to be an exception. Therefore, in many fields 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 accountancy, architecture, medicine, engineering, law, librarianship, nursing, social work and teaching.
Sometimes the professional status of an activity is controversial, such as in whether or not professionals should be allowed to compete in the Olympic Games.
It has been suggested that the crude, all or nothing categories of professional or amateur should be reconsidered. A historical shift is occurring with the rise of Pro-Ams, a new category of people that are pursuing amateur activities to professional standards.