Milestones, which were mentioned in an ealier post, have now been fully explained by a post on the StarCraft II blog.

Now, you might be asking yourself "What are milestones?" 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.

For the full blog post, click here.

Source:
Battle.net - Season 1 Milestones