First to understand this, let's go over the definition of work.
Now as a noun, work is exertion or effect directed to produce or accomplish something; labor; toil. It can also mean to be productive or operative activity.
Work is the effort put in to attain a certain result, and after you have mastered the subject for which you work, you are considered a professional. As a result, when people ask what you do, they are effectively asking what trade you are in. How skilled you are at your craft and what position you play in the company. You are categorized as a hobbyist if you are really talented at what you do but do not trade your abilities for anything monetary in exchange for the effort you put in. When people ask you what you work as, they usually consider how much money you make and how respectable your job skills is from trade.