FIFA laws of the game:

Handling the ball Handling the ball involves a deliberate act of a player making contact with the ball with his hand or arm. The referee shall take the following into consideration: • The movement of the hand towards the ball (not the ball towards the hand) • The distance between the opponent and the ball (unexpected ball) • Position of the hand does not necessarily mean that there is an infringement • Touching the ball with an object held in the hand (clothing, shinguard etc.) counts as an infringement • Hitting the ball with a thrown object (a boot, shinguard etc.) counts as an infringement Disciplinary sanctions There are circumstances when a caution for unsporting behaviour is required when a player deliberately handles the ball, e.g. when a player: • deliberately and blatantly handles the ball to prevent an opponent gaining possession • attempts to score a goal by deliberately handling the ball A player is sent off, however, if he prevents a goal or an obvious goalscoring opportunity by deliberately handling the ball. This punishment arises not from the act of the player deliberately handling the ball but from the unacceptable and unfair intervention that prevented a goal being scored.

EDIT: I should have used the word "deliberate" instead of "intentional", but otherwise, I was dead on. The only reason to give someone a yellow for a handball would be to prevent an opponent gaining possession or attempting to score.