Profession
(Varies by Type)
A profession is something you can get paid to do, usually closely tied to a Knowledge skill or skills that you have. In addition, you will usually receive a positive Flip Shift when you attempt to use your profession based on the skills you have. You may even be able to convince your GM that another skill you possess (usually with a specialization) can allow you to gain this bonus. For example, it makes sense that someone who wants Profession: Acting would get the bonus if he also took Persuasion [Lying].
Professions are wide open. They can be as specific or as general as the player and GM want.