As taken from the US SC2 forums:
Milestones are seasonal rewards that will be provided to ladder players in all of our StarCraft II: Wings of Liberty leagues. Each league will have four notable milestones. Bronze, Silver, Gold, Platinum, Diamond and Master leagues will have milestones for the top 100, top 50, top 25, and top 8 players in each league division. For Grandmaster, milestones will be awarded to the top 200, top 100, top 50, and top 16 players. These milestones will each have their own unique icon (shown below) representing the league in which the milestone was earned and the actual milestone achievement.
When patch 1.3 is released, all players will be locked into their current league and division. You'll then be given one week to play out your remaining bonus pools (while promotions and demotions into and out of divisions will be disabled, you may still be able to move up or down within that division's ladder). Once the one-week "bonus pool" period is over, Season 1 will officially end and all ladder rankings will be finalized. Milestones will be awarded to the top players in each league division shortly after and we'll display the results in-game in the Leagues & Ladders section. This same process is expected to take place at the end of each season. (In a future patch, we also plan to add a dedicated page in the game UI that will save past performance history so that players can show-off all of their milestone achievements.)
Screenshots for the milestones are below.