The current league divisions are
Bronze
Silver
Gold
Platinum
Diamond
Placement starts off your hidden MMR score (your Match Making Rating, which is used to pair you as evenly as possible) and randomly matches you against players that are available. You can generally predict where you place based on how many wins or losses you get, which is something like
Bronze - 0-2 wins
Silver - 1-3 wins
Gold - 2-4 wins
Platinum - 4-5 wins
Diamond - 5 wins against high-ranked players
Basically, doing particularly well or poorly in one division will eventually move you up or down. The ladder system attempts to keep your win-loss ratio 50/50, so if you start to go significantly one way or the other then you have a good chance of being promoted or demoted. In terms of true competitive tournament play, nobody below Diamond actually matters (read: anyone who isn't Diamond will probably not make it past three rounds of MLG). However, when comparing absolute player levels, since the scoring system is not zero-sum, there is such a thing as "points inflation" and thus the "high diamond" level is constantly shifting upward. In September, it was 2000. Now the top players are pushing 3500-4000.
Blizzard plans to eventually implement a Master and Grand Master division. These divisions will actually carry meaning, because unlike the previous ones, they have strict requirements to reach them:
Master League is limited to the top 5% of players in each Region (North America, Europe, Korea, Southeast Asia).
Grand Master League is limited to the top 200 players in each Region.
Bronze
Silver
Gold
Platinum
Diamond
Placement starts off your hidden MMR score (your Match Making Rating, which is used to pair you as evenly as possible) and randomly matches you against players that are available. You can generally predict where you place based on how many wins or losses you get, which is something like
Bronze - 0-2 wins
Silver - 1-3 wins
Gold - 2-4 wins
Platinum - 4-5 wins
Diamond - 5 wins against high-ranked players
Basically, doing particularly well or poorly in one division will eventually move you up or down. The ladder system attempts to keep your win-loss ratio 50/50, so if you start to go significantly one way or the other then you have a good chance of being promoted or demoted. In terms of true competitive tournament play, nobody below Diamond actually matters (read: anyone who isn't Diamond will probably not make it past three rounds of MLG). However, when comparing absolute player levels, since the scoring system is not zero-sum, there is such a thing as "points inflation" and thus the "high diamond" level is constantly shifting upward. In September, it was 2000. Now the top players are pushing 3500-4000.
Blizzard plans to eventually implement a Master and Grand Master division. These divisions will actually carry meaning, because unlike the previous ones, they have strict requirements to reach them:
Master League is limited to the top 5% of players in each Region (North America, Europe, Korea, Southeast Asia).
Grand Master League is limited to the top 200 players in each Region.