A Challenge is very basic. It is just that: a challenge. Because there are different forms a challenge can take, there is no hard-core system. But what basically happens is I give a challenge for you to solve/learn/etc.
Usually, it will have rules. While it will be perfectly fine with me if you ask me a question via Email, be sure to check the rules before you do so. I try to be as thorough as possible (with the one challenge that exists so far).