This stands for electronic voice phenomenon, it is the alleged communication of spirits which as been recorded on tape or heard through other electronic devices. In most circumstances voices or sound can not be heard instantly, but on playback they can.
EVP was first discovered by Fredrich Jurgenson who was recording bird song in the Swedish countryside, on playback he heard a mans voice talking about bird songs and instructions on how to record more voices. He only heard the voice on playback, but nothing while recording the tape. To try and do this yourself it is best to go to a location that is known for being haunted, you will need a tape recorder with an external microphone, so there will be no direct disturbance, try to be somewhere where you will not be disturbed or with people you trust. Set up the equipment with the microphone being carefully placed away from the recorder, and use a brand new tape, then you can do either one of two things, but before either of them, make a note of the things you can already hear around you, so you know if they are heard on the tape, the first is to leave the recorder going on its own for however long you want, and then see if you have captured anything, the other is to stay with the recording and ask questions remembering to give a small space in between, just incase there as been a reply