A readied action is an Immediate Reaction. A charge is a movement with an attack at the end.
Suppose the following case:
A PC charged with protecting his party's wizard does not want to move away from the wizard so he readies an action triggered if anyone moves adjacent to the wizard. Later in the round the wizard is charged by an enemy thus triggering the conditions of the readied action.
Does the attack take place before or after the attack portion of the charge is resolved?
It depends on the trigger of the readied actionDDI.
As already cleared out on the Player's Handbook page 291...
Interrupting an Enemy: If you want to use a readied action to attack before an enemy attacks, you should ready your action in response to the enemy’s movement. That way your attack will be triggered by a portion of the enemy’s move, and you will interrupt it and attack first. If you ready an action to be triggered by an enemy attack, your readied action will occur as a reaction to that attack, so you’ll attack after the enemy. Note that an enemy might use a power that lets it move and then attack. If you readied an action to attack in response to that enemy’s movement, your readied action interrupts the movement, and you can attack before the enemy does.
Charge is movement, then attack. By tuning your trigger on the movement part ("as soon as an enemy is within reach") you are able to use an immediate reaction to hit the charging monster before the wizard is attacked.
