Well I don't know much about the components, but I might be of some help with the concepts here...
Accelerometers do just what their name implies... they measure acceleration (as a swiping gesture with a Wiimote, or a car crash)... They can, however, also be used to measure orientation to some degree, since gravity is a constant force, and a sideways accelerometer will read different than an up-standing one...
A gyroscope, on the other hand, is a device for measuring orientation, since gyros spin really fast and spinning objects tend to retain their initial orientation, this can be used to measure relative orientation (and even absolute, if you have a gyro that never becomes misaligned)
The amount of measurements an accelerometer or a gyro can report is stated in degrees of freedom.
A degree of freedom is an axis about which you can displace yourself... like moving left/right, or pitching up/down...
In 3D space there can be 6 degrees of freedom... those are left/right, up/down, fore/aft (XYZ translations), Pitch, Yaw and Roll (XYZ rotations)
However, one might come across devices that say they can measure more than 6 DOF, like the 9DOF sensor from SparkFun... that doesn't mean it can measure hyperdimensional space (bummer)... but simply that it can measure absolute as well as relative displacements.
Now, on the difference between relative and absolute measurements...
Relative measuring is basically saying how much you displaced yourself from your previous measurement... no information of where you are can be known without a first (absolute) reference.
Absolute measuring is exactly the other part... it tells you where you are in relation to a wider scope (like magnetic north, or a fixed point in XYZ space), but how much you've displaced yourself since the last measurement would have to be calculated later.
So a device that states 9 degrees of freedom is probably an accelerometer coupled with a gyro and a compass for 6DOF relative measurement and absolute measurement of the 3 orientations so it gives you translation XYZ, pitch, yaw, roll, azimuth, zenith and... absolute roll (does it have a name?)
Now, on this notion, the maximum amount of degress of freedom one could have in 3D space would be 12, and include absolute and relative translations and orientations... Absolute position tracking is tricky, because it requires a frame of reference, which most people aren't willing to build on their living rooms... so a small 12DOF sensor is not something you can pick up at SparkFun yet, but if you do find one, please let me know so I can buy a thousand
I hope this helps... (and I hope I didn't write too much nonsense)