From United States Soccer Federation Publication

The offense known as "handling the ball" involves deliberate contact with the ball by a player's hand or arm (including fingertips, upper arm, or outer shoulder). "Deliberate contact" means that the player could have avoided the touch but chose not to, that the player's arms were not in a normal playing position at the time, or that the player deliberately continued an initially accidental contact for the purpose of gaining an unfair advantage. Moving hands or arms instinctively to protect the body when suddenly faced with a fast approaching ball does not constitute deliberate contact unless there is subsequent action to direct the ball once contact is made. Likewise, placing hands or arms to protect the body at a free kick or similar restart is not likely to produce an infringement unless there is subsequent action to direct or control the ball. The fact that a player may benefit from the ball contacting the hand does not transform the otherwise accidental event into an infringement. A player infringes the Law regarding handling the ball even if direct contact is avoided by holding something in the hand (clothing, shinguard, etc.).
The rule of thumb for referees is that it is handling if the player plays the ball, but not handling if the ball plays the player. The referee should punish only deliberate handling of the ball, meaning only those actions when the player (and not the goalkeeper within his own penalty area) strikes or propels the ball with his hand or arm (shoulder to tip of fingers).
For purposes of determining deliberate handling of the ball, the "hand" is considered to be any part of the arm-hand from fingertip to shoulder. Using the top of the shoulder is not considered as using the hand.