A quick search of your online documentation will tell you all about Quaternions:
---from the online manual (4.5)--------->
Maya provides smooth quaternion interpolation of animated rotations. This prevents the gimbal lock and flipping problems that are sometimes encountered with Euler angle interpolation. Maya also supports Euler angle interpolation for cases where you prefer Euler interpolation, and for pre-4 Maya scene files.
The quaternion curves are synchronized, which means the keyframes on the XYZ curves that determine the rotation stay locked together in time. When you add, delete, or move a keyframe on one curve, the corresponding key is also updated in the related curves. This eliminates unexpected interpolation problems that can occur when keyframes are deleted from only one of the axes, or vertices are moved independently in time.
Maya also has an option for creating Synchronized Euler-Angle Curves. In this mode, the keyframes are locked together as in Quaternion but the interpolation between keyframes is in Euler angles.
You can change the rotation interpolation type of existing curves using the Curves > Change Rotation Interp menu in the Graph Editor and Dope Sheet.
Regardless of whether the rotation is quaternion or Euler angles, the Channel Box displays Euler angle values.
To set rotation interpolation for new curves:
Select Window > Settings/Preferences > Preferences and select the Keys Category under Settings.
Under Rotation Interpolation, set New Curve Default to one of the following:
Independent Euler-Angle Curves
Synchronized Euler-Angle Curves
Synchronized Quaternion Curves
I know the online manual pre 4.5 is pretty shoddy, so I cut and pasted the above.
hope this helps explain a bit... im sure its in the 4.0 docs someplace too!
cheeers