What are X, Y and Z referring to in Rigidbody and Configurable Joint?

I have coordinate axes on the top right corner of the scene window, x, y and z. In Configurable Joint, I have a local axis that will define the object's natural rotation based on physics simulation. So I assume that what it means is that I put a 1 in the axis' name it should rotate around and a 0 where it shouldn't. But that wasn't true. Same applies to Constraints in Rigidbody.

In general, what are X, Y and Z referring to in Rigidbody and Configurable Joint? I am talking about XMotion, Angular XMotion, XLimit, Axis-X, etc...

As most people seem to speak german here, reply in german if you can.