What's a QSL card?
"QSL cards confirm either a two-way radiocommunication between two amateur radio stations or a one-way reception of a signal from an AM radio, FM radio,television or shortwave broadcasting station. They can also confirm the reception of a two-way radiocommunication by a third party listener. A typical QSL card is the same size and made from the same material as a typical postcard, and most are sent through the mail as such." Source: http://en.wikipedia.org/wiki/QSL
More here: http://d.lib.ncsu.edu/collections/catalog?f%5Bsubject_facet%5D%5B%5D=Amateur+...
Found here: http://www.presentandcorrect.com/blog/qsl