See Also: UIAcceleration Members
The iOS accelerometer (see UIKit.UIAccelerometer) reports acceleration events as a triplet of vectors. Looking at the face of an iOS device, positive X is to the right, positive Y is to the top, and positive Z is towards the viewer. The value of each vector is a double whose units are g-force.
Application developers should not rely on the accuracy of a single acceleration event but rather should rather average or otherwise interpolate from a series of readings.