In most modern smartphones there is a feature called NFC, which is short for Near Field Communication. This tech uses electromagnetic induction with two loop antennas and the information is transferred on a radio signal in the 13.56 MHz band. If a device approaches another device this is called peer-to-peer sharing and both of the devices generate RF fields to transfer data. But in the case of a NFC tag, the tag does not have its own power supply, therefore it uses inductance from the active device and then transfers a RF signal.
There are many applications for this technology that anybody with a phone can do. Currently anybody with a relatively new iPhone or Android can use NFC to wirelessly pay were it is available. But if you have an Android and some NFC tags around you can program them to carry out simple or complex functions.
The tag pictured above, only about 3.5 cm2, has been programmed to give the wi-fi password to anybody that touches their phone to it.
Down below you can see the inside of the tag thanks to it being thin.