QSL cards are postcards exchanged between amateur ham radio operators to confirm contact between DX radio stations. Millions of QSL cards have been exchanged among ham radio operators, and many people collect old and interesting QSL cards.
QSL cards always show the callsign of the station, and usually include the operator's name and address. They often have photographs of the operator or a picture of his or her country or location, which is part of what makes them so attractive; a QSL card is a colorful souvenir from a remote or interesting location. Radio operators often post QSL cards in their radio shacks, especially cards from extremely remote DX stations, or stations in interesting places, or cards from famous amateur radio operators such as the late King Hussein of Jordan, whose QSL cards from station JY1 were highly valued by ham radio operators.
The cards usually include details of the contact, such as the callsign of the other ham radio station, the date and time of the contact (usually in GMT), and the frequency or band on which the contact was made. The signal report often includes information about the amateur radio equipment that was used, such as the ham radio transceiver (or the transmitter and receiver) and the ham radio antenna.
QSL postcards are often simply placed in the mail, from one ham radio operator to another, but many countries have a QSL Bureau, which collect cards from operators in their country, and then mail them in bulk to the QSL bureau of another country. Obviously, using a QSL bureau is a much slower way to send or receive cards, but it is also a lot less expensive than the postage for individual cards, and can be more reliable in countries where mail service is not very dependable.
The term QSL itself is one of the classic radio "Q Codes" and can be both a request ("Can you confirm receipt of my transmission?") and a response ("I confirm receipt of your transmission.")
A list of Famous Radio Rooms and Call Signs
A list of QSL Bureaus
A list of Amateur Ham Radio Associations