Implements are items wielded by certain characters to channel their powers. Your class description or a feat tells you which implements you can wield, if any. To use the powers and the properties of a magic implement, you must be able to wield that implement.
The implement keyword identifies a power that can be used through an implement, and the implement must be a type wielded by the power’s class.
You can use implement powers without an implement, and wielding a nonmagical implement confers no benefit. If you wield a magic implement, you can add its enhancement bonus to the attack rolls and the damage rolls of implement powers you use through it
Published in Player's Handbook 2, page(s) 220, Player's Handbook 3, page(s) 219.