From Liquipedia Starcraft Wiki
The term "progaming" is an abbreviation of "professional gaming". What these terms refer to is the act of people playing video games and making a living by doing so (usually by being paid monetarily). The concept is similar to professional sports. Usually, progaming is used as a form of entertainment and is broadcasted or recorded in some way for the enjoyment of viewers. An example of progaming is the situation in South Korea regarding StarCraft. In South Korea, certain players of high skill at the game of StarCraft are recruited onto teams to practice and compete with their teammates for many hours a day. These people are paid to do this and are referred to as progamers. An example of a progamer is Bisu.
 Korean Progamers
In the Korean Brood War progaming circuit, the term progamer has stricter connotations than in other games. Aspiring players who may be skilled but not involved in the professional scene are known as amateurs. An amateur who wants to enter the professional scene must first gain a semi-professional license from KeSPA. Although the rules for acquiring this license have changed slightly over the years, one must either get top place in the Courage tournaments or in rarer cases be gifted a license from a pro-team to become a semi-pro. The professional teams then evaluate semi-pros and may choose to draft one onto their team, at which point they are considered a progamer and can play in KeSPA leagues. Progamers are differentiated by their place on the A-team, which is the roster of players that can play in Proleague matches, or the B-team, which serves as practice partners for the A-team and plays in the minor Dream League. Both A and B team members can compete in the individual leagues (OSL and MSL). Based on their performance and in-house ranking tournaments, progamers are shifted between the A and B teams by their coaches on a monthly basis.
Occasionally the term "S-class progamer" is thrown around, but it is merely a subjective term to indicate an A-team player who is playing superbly at the time. Similarly, a player who shows exceptional domination and impact on the scene may eventually be recognized as a Bonjwa, but there is no quantitative criteria for it.