The QSL
"QSL" is one of the "Q signals", part of an international set of radio communication codes. It means "I acknowledge receipt". There are many Q signals, which help amateurs communicate quickly in a language-neutral manner.
QSL cards are traditionally exchanged after a meaningful "QSO" (radio contact). QSLs are collectible in their own right, but they also serve as proof of contact. This is useful when applying for special awards, such as "Worked All States" or "DXCC" (worked 100 or more countries). Sometimes, "SWLs" (shortwave listeners) will send a card confirming reception of a QSO in progress between two amateurs.
For some versions of my own card, see The AA6E QSL Card.
For a book about a particular collection of QSL cards, see "Hello World", written by Danny Gregory and Paul Sahre, published 2003 by Princeton Architectural Press.
In recent years, all-electronic substitutes or supplements for traditional QSL cards have developed. The leading services are eQSL.cc and ARRL's "Logbook of the World". They have different goals and styles. eQSL maintains the idea of a QSL card image, which is stored and transmitted over the Web. "LOTW" focuses on hams' logbooks which are compiled into a central database and cross-matched for the purpose of scoring contests and verifying certificates, such as DXCC.
QSLs received at AA6E
My collection is more modest than some, but it shows the wide range of styles and places of origin. At this time, I only show the front side of most cards. Many cards are two-sided, however, with the reverse being used for the contact details: date, time, frequency, etc.
The collection is available on my Flickr site.