Electronic voice phenomena or EVP is the appearance of mysterious voices on tapes or recordings. They are usually hard to make out, ambiguous and hidden among the static, although some claim they are voices of spirits trying to communicate with the living.
Others claim this is a result of apophenia, a psychological tendency to see meaning in noise where there is none.
There’s been quite a bit of interest in this lately, probably spurred by the upcoming release of the movie White Noise, which has EVP recordings as its major plot device.
The Scotsman has an article exploring the phenomena from several angles, including quotes from a psychologist and parapsychologist on approaches to understanding these puzzling communications.