Magnetic induction is a technology that you will probably remember from your physics classes at high school.
You need two coils, a transmitter coil and a receiver coil. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery.
The presentation below shows how to build a wireless charging system using magnetic induction.
You will see that products need more than coils and alternatining currents. For energy-efficient power transfer the phone must be able to shut down the transmitter when the battery is full. The phone, therefore, needs to send control signals to the charging station.
The presentation is also available as PDF download.