Wikipedia says: QSL, or QSL card, is the confirmation of a QSO (a radio contact) between two radio amateurs. “QSL” is a Q code, which means “I confirm contact with you.” A QSL card is a (usually postcard-sized) hardcopy, containing the specific details of a QSO. It usually contains the callsign of both operators, the time and date of the QSO (usually in GMT), the radio frequency used, the mode of transmission used, and RST (Readability, Strength, Tone) reports exchanged. RST is a numeric code, that indicates how well (or badly!) the radio signal was received. Sometimes the QSL card will contain an image, perhaps of something associated with the operator’s home town. QSL cards are very important to the radio amateur since they confirm that a QSO took place and are used as proof when applying for a Ham Award.
… and a little more information from Amateur Radio Victoria: A QSL card contains some basic information – the amateur station’s callsign, location, licensee’s name and postal address, and often details about the amateur station equipment. It will also include details of the contact, the date/time, frequency, mode of transmission, and signal report. QSL cards should not exceed 140mm by 90mm or be less than 125mm by 80mm. They must not be printed on lightweight paper (such as 80gsm copy paper). Preferred weight is in the range 120 -180gsm. The callsign of the station to receive the card is written on the top right hand corner. The card size and placement of the recipient callsign were made standard worldwide some years ago to make it easier to sort and handle large quantities of cards. The QSL Bureaux only exchange cards between themselves, and there are a very few countries which don’t have a bureau. In these cases, radio amateurs wanting a card from a non-bureau country must QSL direct by using the normal postage system.
… I’m working on my own QSL card.