Device Orientation Demo
Accelerometer:
The accelerometer detects when the device is in motion. This would be useful for games or things when moving the device is important.
Movement horizontal is X, vertical movement is Y and movement forward and backwards is Z. It only detects force, not orientation. So, it'll only change while you're moving the device around.
Read this for more information.
Gyroscope:
The gyroscope detects orientation and is measured in degrees. Changes in alpha is like swinging the device like a pendulum on a grandfather clock, rotation around the Z axis. Beta is rotation around the X axis. So, beta is like if you have the device laying on the table face up. If you pick up the end with the mic, but leave the end with the speaker down (like you're opening a lid to something), you're rotating beta. Gamma is twisting the device. It's rotating around the Y axis. Again, if you lay the device on a table and pick up the right edge, but leave the left edge on the table, you're changing gamma.
Read this for more information.
Compass:
Believe it or not, iOS devices also have tiny compasses in them so they know which direction you're pointing. 0° and 360° are north. 180 is south. Etc.
Read this for more information.