In your cell phone charger, there is something called a transformer, which consists of two coils of wire. One of those coils always has electricity running through it when it is plugged in. The other coil, which is plugged into your phone, will allow electricity through it by something called inductance, since the coils are not directly connected. This creates a load on circuit, and thus requires power from your plug on the wall. When you unplug your cell phone, electricity can not circulate through one of the coils. However, there is still electricity running around the other coil. Since the wires in that coil have a finite resistance, it ends up drawing power from the wall, and the energy goes into heating up the wires. If you've ever felt the charger after its been plugged in for a while, it should feel warm because of this.

The power that is released across a resistor in a circuit is released in the form of heat that heats up the circuit. Thereafter, the energy is dissipated into the environment.

When cell phone chargers are plugged into the wall, they are accessing the city's electricity "grid". Because of this, at the end of a cell phone charger there is a "potential". This potential (also called "voltage") means that as soon as you connect something to it, such as a cell phone, electricity will flow through. Flowing electricity is called a "current".

When a charger is plugged into the wall with no cell phone plugged into it, there cannot be a current flowing because there is nowhere for the electricity to go. The circuit is not complete, so no electricity is being used, and no electricity is being wasted. Having said that, there may be a tiny leak of a little bit of electricity through the charger. If this happens, the energy would be dissipated as heat and the cell phone charger would warm up. Energy cannot just disappear; it does have to go somewhere. So, unless your cell phone charger is warmer than the air in the room, it is not wasting energy to have it plugged into the wall.