touches-canceled

What is the touches canceled event, and how is it different from touches ended

I just don't understand what apple means by the touches canceled event and how is it called. ...

Querying current number of touches on screen without using events on iPhone

I have an application that starts playing a sound when user touches the uiview and changing to different tones as the user slides the finger on the screen. The sound stops when the user lifts the finger. I am using the touchesBegan, Moved and Ended Events for this. My problem is touches Ended (and/or cancelled) is sometimes not fired ...