Numbers stations (or number stations) are shortwave radio stations of uncertain origin. In the 1950s, Time magazine reported that the numbers stations first appeared shortly after World War II. Numbers stations generally broadcast artificially generated voices reading streams of numbers, words, letters (sometimes using a spelling alphabet), tunes or Morse code. They are in a wide variety of languages and the voices are usually female, although sometimes men's or children's voices are used. Evidence supports popular assumptions that the broadcasts are used to send messages to spies. This usage has not been publicly acknowledged by any government that may operate a numbers station.
It has been reported that the United States uses numbers stations to communicate encoded information to persons in other countries. According to the notes of The Conet Project, which has compiled an impressive number of recordings of these transmissions, numbers stations have been reported since World War I. If accurate, this would make numbers stations among the earliest radio broadcasts.
It has long been speculated, and was argued in court in one case, that these stations operate as a simple and foolproof method for government agencies to communicate with spies working undercover. According to this theory, the messages are encrypted with a one-time pad, to avoid any risk of decryption by the enemy. As evidence, numbers stations have changed details of their broadcasts or produced special, nonscheduled broadcasts coincident with extraordinary political events, such as the August Coup of 1991 in the Soviet Union.
Although no broadcaster or government has acknowledged transmitting the numbers, a 1998 article in The Daily Telegraph quoted a spokesperson for the Department of Trade and Industry (the government department that, at that time, regulated radio broadcasting in the United Kingdom) as saying, "These [numbers stations] are what you suppose they are. People shouldn't be mystified by them. They are not for, shall we say, public consumption.
These stations still operate and exist in numerous countries around the world. You can listen to samples from The Conet Project here: http://www.archive.org/details/ird059. Addionally, many websites are dedicated to finding and sharing these stations. Check them out if you have some radio equipment. - Josh