The cow coordinate system is a right handed coordinate system, where - while looking on the screen - the x-axis points to the right, the y-axis points downwards and the z-axis point into the screen away from the user.
The rotation of an object is defined as a counter clockwise rotation. However, since the coordinate system is viewed from the back, the rotation appears as what is normally known as clockwise.
Shown is the how an image is placed into the coordinate system.
Left for 2D images, right for 3D images.
The rotation center is marked in the middle of the image.
ZXZs
That is: