A profession is a specialized work function within society.
A profession is always held by a person, and it is generally that person's way of generating income. Some historians believe that the foundation of modern civilization is division of labour into different professions, thus increasing the level of expertise held by professionals.
'Profession' is very often restricted to include only those occupations requiring extensive study, such as law, medicine, the Church or engineering. In which case other persons who generate their own incomes engage in trades or occupations, or even jobs. One refers to their work not as "professional[?]", but using a more generic term such as "occupational". Such usage avoids the confusion caused by vague use of the words "professional" and "professionalism" to express undefined approval or a sense of exclusiveness. Note that sociologists have been known to define "professionalism" as organised exclusivity along guild lines, much in the sense that George Bernard Shaw characterised all professions as "conspiracies against the laity".
See also: list of professions