A profession is a set of trade-oriented skills that players may use to gather, create, or enhance items. There are two categories of professions. The first is primary; the second, secondary. There is another subcategory that acts like a profession, but it isn't listed in either of the before mentioned categories. You learn professions by skill trainers that teach the craft.
Categories for Professions
Primary professions are professions that a player can only choose two of. A player can unlearn the primary professions, so it will free up space for a player to choose a different one. Each of these professions is separated into further categories: Gathering and Production Professions. Most people in the game will call Production Professions simply crafting or building.
Gathering (Supply and Demand) professions require you to go out into the world and gather materials for the production professions. For players who like to be self-efficient, these materials will be directly used by them. However, there are some players who will sell these items on the auction house to other players who are skilling up their Production Professions without gathering the items themselves.
Items that are created as a result of the Production Professions will either be used right away, saved for a recipe that needs the item, or sold on the auction house.
Secondary Professions are professions that a player can take all of. There are four secondary professions as of right now.
With several expansions, new professions have been introduced. The jewelry profession may only be learned on accounts that have bought the Burning Crusade and Archaelogy may only be learned by those accounts that have Cataclysm while inscription was introduced in a patch.