Special symbols shown next to your reputation, which provide indicators that a user has met a specific achievement.
Badges are icons you can collect, shown next to your reputation and profile picture. There are 3 categories of badge: bronze, silver and gold. Harder badges are silver and badges that require a large amount of effort are gold.
The complete details of how to earn each badge can be found here.
This tag should be added to questions that are specifically about the badge system, or specific badges. feature-requests requesting new badges should be posted to Mother Meta, as badges are not a site specific feature. Posts asking "How does this badge work/How do I get this badge?" are off-topic, and should be referred to the above Mother Meta post.