What does chucking or throwing in cricket mean ?

What is chucking , what is the difference between throwing the ball and bowling it ? How do you know if a bowler is throwing or bowling ? This throwing saga has haunted cricket in the past year but what is throwing really and why is it not allowed ? How does a bowler get an extra advantage by throwing ? Why is it that most of the times off spinners and fast bowlers are accused for throwing why not leg spinners ? What does arm straightening mean ?