Cognitive Radio (also known as "Opportunistic Spectrum Access") was first coined in an IEEE journal and is now considered the holy grail of communications research by many electrical/communications engineers.
To understand cognitive radio one must first be familiar with software radio. The operating parameters of a traditional radio (center frequency, modulation type, bandwidth, etc.) are defined in hardware and static in type. A software radio is a device which, in affect, brings the "software to the antenna" i.e. replaces the encoder/modulator/awgn/slicer with software. This allows much more flexible radio devices as they can use any frequency, with any modulation type, etc.
The next thing we must understand is that spectrum is scarce and increasingly expensive. The FCC's old spectrum licensing paradigm of fixed frequency assignment is outdated and can easily be improved. Here's an example: Verizon Wireless ownes (say) 1800 MHz nationwide and at all times but, if I could ensure little to no interference with Verizon's operations in the middle of the desert or at 4AM, shouldn't I be able to sub-license (or sub-lease) the spectrum?
This is where cognitive radio comes in: they scan the spectrum looking for "holes" (barely used frequencies), adjust their center frequency accordingly, find the best modulation type, etc. and transmit/receive at these frequencies. This will open up a lot of spectrum (the FCC noted spectrum utilization typically varies between 15% and 85%) and decrease the cost of spectrum access (or make the FCC a ton of money either way). The problem is that the engineering challenges are formidable (hidden terminal problem, collaborative sensing, etc.) and expensive (if we make a mistake, me might knock Verizon off the air) but eventually fixed licensing will be a thing of the past and we'll have devices that will operate at whatever frequency/modulation type/etc. they determine best and pay per usage (or some similar model).
For more info:
(The Technical Committee on Cognitive Networks (TCCN) of the IEEE Communications Society)
(IEEE Cognitive Radio Information Center)