How to interpret Rosaria Odomerty readings?

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Hi everyone, I am new to ROS and robotics. I had a few question about ROSARIA and Pioneer 3dx robot. I am following the tutorial through which I came to know how to move the robot and get readings out of it. I used rostopic echo /RosAria/pose to get the position and orientation. I understand the position part but I don't understand the orientation part. Can you you help me understan the orientation part. I think the value is in radians but I don't understand how it changes with respect to robot position. Also I don't understand the difference between orientation z and w. I have attached a set of readings :

header: 
  seq: 488
  stamp: 
    secs: 1466895723
    nsecs: 5427791
  frame_id: odom
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: -0.488
      y: 0.662
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: -0.802976539096
      w: 0.596010635527
  covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
twist: 
  twist: 
    linear: 
      x: -0.102
      y: 0.0
      z: 0.0
    angular: 
      x: 0.0
      y: 0.0
      z: -0.181514242207
  covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
---
header: 
  seq: 489
  stamp: 
    secs: 1466895723
    nsecs: 105432307
  frame_id: odom
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: -0.485
      y: 0.671
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: -0.809313639061
      w: 0.587376739095
  covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
twist: 
  twist: 
    linear: 
      x: -0.098
      y: 0.0
      z: 0.0
    angular: 
      x: 0.0
      y: 0.0
      z: -0.195476876223
  covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0 ...