GNU radio is a free software toolkit for building and deploying software radios; for learning about DSP and communication systems; for creating new kinds of radios, modulations, protocols, development environments; regulatory hacking; exploring.
I'm in it especially for the new kinds of modulations, by the way.
Software radio is: Get the software close to the antenna; turn hardware problems into software problems; defines the waveforms transmitted; demodulates the waveforms received.
The equipment is off-the-shelf hardware. Conventional radio stuff? Spectrum monitoring? Multi channel, mode, morph on the fly, better spectrum utilization, cognitive radio.
With a software model, you have one RF frontend and you suck down an entire spectrum of broadband; it lets you do distributed, parallel multiprocessing of an entire range of frequencies.
AI! Smart radio detection.
What kind of hardware? Narrow: Soundblaster, narrow band RF front end (NFM, USB/LSB, AM, ...); audio output from receiver (packet, PSK31, ...); sub 50 kilohertz. Wide band: high speed A/D (20M+ samples/sec), wide band RF front end (TV tuner).
Politics: MPAA / CPTWG / BPDG "Broadcast Flag". FCC software radio in general; FCC free software radio.