I pretty often hear SC2 shoutcasters talking about code A and code S. What is it, exactly ? A league ? Some sort or special ranking for grandmaster ranked players ?
They are tiers of the Global Starcraft League, held by GomTV in Korea. Players who qualify for Code A (which is inferior to Code S) are considered vastly superior to your average Grandmasters player, even if you can generally find them in GM on the ladder.
You will also hear them mention when someone is or was a Code S champion. This title is considered the biggest there is to win in the SC2 scene, and hence, the current and past champions are always big contenders at other events.