Qi is an interface standard for inductive energy transfer of the "Wireless Power Consortium". Members of this consortium include, without limitation known companies such as HTC, Samsung, Nokia, Sony, and many others.

How does wireless charging work?
For inductive charging requires two devices. A receiver (Qi compatible device) and a transmitter (charging station).
The power transmission is effected via coils which generate a magnetic field. Above this magnetic field, the receiver can accommodate up to 5W and thereby recharge the internal battery.

The Qi wireless receiver fits discreetly under your Samsung S4 battery cover and allows wireless Qi charging with any Qi charger (optional)
Easy to install under the battery cover of your phone
Lightweight, ultra-thin (0.5 mm)
Only suitable for SAMSUNG Galaxy S4
Contents: Qi receiver, operating manual