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.