There is a ladder within each skill league.
Players do not directly select which map they play on. Instead, they may vote against three "unpreferred maps" out of a list, and are less likely to play on said map, but are not guaranteed to do so.
Winning a match grants points which can eventually lead to a rating change. When two players are about to face off, each is given a "favored" rating, which determines how many points each will get for defeating the other.
As of April 2012, season 7, Blizzard experimented with "loosening" up the AMM, causing players to be pitted against others with slightly higher and slightly lower ratings. The immediate result was a failure, as players at both the extreme upper and lower skill ranges were being matched with inappropriate opponents. Blizzard introduced changes to correct the situation. At the higher skill levels, the system now chooses more competitive matches over faster matchmaking.
Warcraft III Automatic Matchmaking. Battle.net Accessed 2009-11-07. (Explanation for AMM in Warcraft III)