Wireless Charging is just what it sounds like — a way to charge your phone, tablet or other device without using any wires. Wireless charging is a simple as placing your device on a charging pad and letting the magic happen. The charger itself is of course connected to a power source, but your device doesn't have to be physically plugged in. Wireless charging has actually been around for years and seen in devices like the Palm Touchstone, however people are now most familiar with it in the form of Qi.
More and more Android devices are supporting wireless charging, and it's likely that soon all Android devices will support this method.
If you really want to dive into wireless charging and find out all there is to know, you can read over Wireless charging, in plain English, but here's a quick breakdown:
Wireless charging uses two resonant inductive couplings to transmit low-power signals between two devices. The base station has a transmitter coil and your phone has a receiver coil. The base station regularly sends a signal out, and when a resonance or capacity change is found because a compatible receiver coil is close enough, the signal is modulated and inductive charging begins.
Inductive charging is using two electromagnetic coils to create a magnetic field between two devices — in this case the coils in the charger base and your Android. This is the same theory that the transformer you plug into the wall to charge your phone the normal way uses. A magnetic field "creates" electricity through the difference of potential and vibration.
The coil in your Android is also connected to the battery charging circuit, and your battery is charged using the energy induced in the magnetic field. Of course, excess heat is created as well, and that's part of why wireless charging isn't the most efficient way to transfer power from the wall to your battery. This is why it takes longer to charge your phone on a Qi pad than it does to plug it into the wall. While new methods and materials use higher frequencies and thinner coils than past iterations, wireless charging is still less efficient and more costly than standard charging over a wire.