It's narrow band radio teletype using a 31 cycle signal. PSK31 brings ham radio and computing together for a new way of communications. It's like an internet instant messenger for hams. All you need is your radio, a computer with 16-bit sound card, a simple circuit for interfacing the two and some software. What's best is that the software is free.
Most people transmit on the same frequency.
You must also make sure you don't drive your transmitter too hard. With the small bandwidth, this is low power communications. Just drive the transmitter with minimal voice for a clean signal. PSK uses the upper sideband. This is because of the way the soundcard handles audio.