How do “badges” work?
I have recently joined stack overflow and have 3 badges. How have I been awarded these, and what do they mean?
You can see a list of badges and a brief explanation on the badges page - http://stackoverflow.com/badges
As @chrisF mentioned - you can also click on the badge to go to a page with users that have been awarded that badge and also a short description.
For a more indepth explanation, you can check out this post - List of all badges with full descriptions