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The MIT design consists of two copper coils. One, attached to the power source, is the sending unit. Rather than send out electromagnetic waves, it fills the space around it with a magnetic field oscillating at a particular frequency. The second copper coil is designed to resonate with that oscillating magnetic field. A copper coil within an oscillating magnetic field generates a current, enough, in MIT's case, to power a light bulb. Power transformers make use of something similar, called magnetic induction, to transmit power between coils over short distances. But those coils aren't designed to resonate with each other. Resonant coupling makes the transfer of energy almost a million times more efficient.
Since the magnetic field doesn't radiate, most of the power that isn't picked up by the receiving unit is bound to the originating coil, rather than being lost into the environment. That also means that this system has a limited range, and the smaller the receiver, the smaller that range is.
