The weird world of numbers stations...
Spy numbers stations are stations on HF that transmit messages consisting of groups of numbers.  Sometimes letter groups are sent instead.  Transmissions usually consist of a male or female voice in AM or SSB reading the number groups.  CW stations are also used.  Rarely are callsigns used, and the stations don't use ITU-allocated frequencies.  The transmissions are intended only for a select group of people, so no information about the stations will be released by those who are operating them.  No government agencies will even admit the stations exist. 

The stations are intended as one-way broadcasts to agents in the field.  The agents themselves do not transmit the numbers messages.  The system is simple but effective, and totally secure.  It may seem anachronistic in today's high tech world of high-speed internet and PGP to rely on seemingly random numbers spoken over a shortwave radio.  However, not all parts of the world yet have internet access, and it is very easy to trace email from one point to the other.   It would also attract attention to the agent if he were receiving encrypted email.  It is virtually impossible to trace who is listening to numbers broadcasts, and a simple portable shortwave receiver would not attract very much attention to an agent in a foriegn country as would some high-tech communications equipment.

I first became interested in numbers stations after reading about them in William Poundstone's book
Big Secrets. At the time I wasn't yet into shortwave listening.  I heard my first numbers station while in the Philippines.  It was the V13 "New Star" numbers station on 8300 kHz, but at the time I wasn't aware of what it was I was hearing.  It was a female voice reading numbers in Chinese, but not being able to understand that language, the four-figure groups being read were a mystery to me.  It wasn't until later that I figured out what it was I heard.

Later, while in England, I was lucky to make the aquaintance of Simon Mason, a numbers enthusiast and author of
Secret Signals, The Euronumbers Mystery. He sent me a copy of his book and a tape of different numbers stations.  Secret Signals is an excellent reference book on many European numbers stations and is available online at Simon's website.  Though many of the stations and schedules are now defunct, it makes for interesting reading.
Links
Covert Comms Archive
Chris Smolinski's Spynumbers.com
Simon Mason's Shortwave Espionage site
Enigma 2000
Radio MonitoringHome