Experience is used for "earning" (purchasing) new medals. You get experience through participation on @Z, and gain more based on your member rating.

The following things give experience:
-Posting a reply on the forum
-Posting a new thread on the forum
-Someone else replying to your threads
-Posting a thread that gets flagged news worthy (displayed on the front page)
-Participating in the StarCraft League and finishing games in it (when it's going on)

Some medals are not available for purchase with medals. Instead, they require specific conditions to be met before they are awarded to you. An example is having an ICQ number associated with your account to get the ICQ medal, or a max-level Draenei in World of Warcraft to get the Draenei medal.