According to Wikipedia, QSL cards are exchanged between amateur radio operators “to confirm two-way radio contact between stations. Each card contains details about one or more contacts, the station and its operator. At a minimum, this includes the call sign of both stations participating in the contact, the time and date when it occurred (usually specified in UTC), the radio frequency or Band used, the mode of transmission used, and a signal report. One national association of amateur radio operators, the IARU Region #2, recommends a size of 3½ by 5½ inches (89 mm by 140 mm).”
QSL cards are mostly collected by other ham radio operators. Although they look like a postcard, a QSL is really a ham radio operator’s calling card and are frequently an expression of individual creativity. They are frequently created with individual pride and reflect the sender’s personality or profession. Consequently, the collecting of QSL cards of especially interesting designs has become an add-on hobby to the simple gathering of printed documentation of a ham’s communications over the course of his or her radio career.
Here’s a link to a video of an especially interesing collector
Martin Luther,Australia's King of QSL Cards