Rotate X,Y,Z... and W?

Hello… this might be an uber noob question, but I figure it’s better to ask and learn something new, than to just pretend to understand.
Here it goes…
I’m playing around with rigging in 2.57 and noticed that my bones have a rotate W value.
I can wrap my head around the bone rotating around it’s x,y, or z axis… but what the heck is W?
Can anyone help me understand?
Thanks in advance.

Blender uses so-called “quaternion rotations” for bones. The mathematics are somewhat complicated, but a quaternion basically incorporates a vector with x,y,z coordinates, and w for rotation. Quaternions help avoid problems like gimbal lock (when you rotate an object so 2 rotation axes overlap), are easier to interpolate than matrix rotations, and easier to modify.

Man talk about alien lingo :spin: