Amateur radio (or "ham" radio) is a non-commercial hobby that encourages radio communications and experimentation. Ham operators are licensed by the U.S. Federal Communications Commission or the equivalent authorities in other countries. Historically, amateur radio has attracted technically-inclined people who are interested in building and operating radio receivers, transmitters, and antennas. Today, excellent equipment is available commercially at attractive prices, so hams rarely build their own basic receivers and transmitters. Many interesting technical frontiers remain to be explored, including for example, computer-based radio signaling methods, satellite communications, and new antenna designs.
Amateur radio has a grand tradition from the 1910's onward, but the proliferation of computers, internet technology, cheap long-distance, and the cell phone have fundamentally changed the environment for the hobby. Many of the younger "would-be's" won't be, because of all the alternative outlets.
Still, there are over 600,000 licensed ham radio operators in the U.S., and active organizations like the American Radio Relay League (ARRL), supporting everything from old-fashioned "rag-chewing", to data transmission, to moon-bounce and satellite based communication, and beyond.
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. Wikipedia: QSL.
For some versions of my own card, see my QSL information.
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. (Amazon)
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.
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.
Most of the collection is available on my Flickr site.
I am member #11607 of the FISTS CW Club.
At the right is a photo of my Bencher Paddle, which is a classic but low-cost design for the "iambic" design -- two independent paddle switches for dashes (left) and dots (right). The paddle must be connected to an electronic keyer which actually generates the dots and dashes. Click on images for full resolution.