Thanks for your explanation. You have a talent for explaining things so that I understand them 
jremington:
A gyroscope cannot provide absolute orientations, only relative angles (rates of rotation, actually).
Can I put it this way?
- Accelerometer: rate of change of velocity
- Gyroscope: rate of change of orientation
- Accelerometer: gives me z-axis
- Magnetometer: gives me North (and x-y plane).
What do Magnetometers actually measure, which values does the IMU return? Is it the x,y,z vector of the magnetic field lines at the current position?